ENH: serialization of categorical to HDF5 (GH7621)
This is implemented by storing the codes directly in the table. And a metadata VLArray of the categories.
Query and appending work as expected. The only quirk is that I don't allow you to append to a table unless the new data has exactly the same categories. Otherwise the codes become meaningless.

This has the nice property of drastically shrinking the storage cost compared to regular strings (which are stored as fixed width of the maximum for that particular column).

I bumped the actual HDF5 storage version to current (was 0.10.1). Its not strictly necessary as this is a completely optional feature, but I am adding the sub-group space 'meta' (which FYI we can use for other things, e.g. to store the column labels and avoid the 64KB limit in attrs, their is an issue about this somewhere)

```
In [14]: df = DataFrame({'a' : Series(list('abccdef')).astype('category'), 'b' : np.random.randn(7)})

In [15]: df
Out[15]: 
   a         b
0  a -0.094609
1  b -1.814638
2  c  0.214974
3  c -0.195395
4  d  0.206022
5  e  1.130589
6  f -0.832810

In [19]: store = pd.HDFStore('test.h5',mode='w')

In [20]: store.append('df',df,data_columns=['a'])

In [21]: store.select('df',where=["a in ['b','d']"])
Out[21]: 
   a         b
1  b -1.814638
4  d  0.206022

In [22]: store.select('df',where=["a in ['b','d']"]).dtypes
Out[22]: 
a    category
b     float64
dtype: object

In [25]: store.get_storer('df').group
Out[25]: 
/df (Group) u''
  children := ['table' (Table), 'meta' (Group)]

In [26]: store.get_storer('df').group.table
Out[26]: 
/df/table (Table(7,)) ''
  description := {
  "index": Int64Col(shape=(), dflt=0, pos=0),
  "values_block_0": Float64Col(shape=(1,), dflt=0.0, pos=1),
  "a": Int8Col(shape=(), dflt=0, pos=2)}
  byteorder := 'little'
  chunkshape := (3855,)
  autoindex := True
  colindexes := {
    "a": Index(6, medium, shuffle, zlib(1)).is_csi=False,
    "index": Index(6, medium, shuffle, zlib(1)).is_csi=False}

In [27]: store.get_storer('df').group.meta 
Out[27]: 
/df/meta (Group) u''
  children := ['a' (VLArray)]
```

cc @JanSchulz
cc @bashtage 
cc @shoyer
cc @immerrr
 Not sure what to say here: I've no expertise in pytable, sorry... :-/ 
 Does using VLarray affect performance with compression?  Fixed length strings can be compressed while I think VLArray data cannot.
 @bashtage I actually changed this back to a regular Array, really for more 'visibility', e.g. you can actually inspect these objects, whereas a `VLArray` objects get pickled. I don't really think their is any actual perf issue. This is just a single array of the `categories` and compared to the size of a table usually is much much less.
 this allows future expadiblity because the array can then be 2-d for example
cc @shoyer

```
In [2]: s = Series(list('aabbcdedfab')).astype('category').to_hdf('test.h5','s',mode='w',format='table')

In [3]: !ptdump -avd test.h5
/ (RootGroup) ''
  /._v_attrs (AttributeSet), 4 attributes:
   [CLASS := 'GROUP',
    PYTABLES_FORMAT_VERSION := '2.1',
    TITLE := '',
    VERSION := '1.0']
/s (Group) ''
  /s._v_attrs (AttributeSet), 15 attributes:
   [CLASS := 'GROUP',
    TITLE := '',
    VERSION := '1.0',
    data_columns := ['values'],
    encoding := None,
    index_cols := [(0, 'index')],
    info := {1: {'type': 'Index', 'names': [None]}, 'values': {'ordered': True}, 'index': {}},
    levels := 1,
    metadata := ['values'],
    nan_rep := 'nan',
    non_index_axes := [(1, ['values'])],
    pandas_type := 'series_table',
    pandas_version := '0.15.2',
    table_type := 'appendable_series',
    values_cols := ['values']]
/s/table (Table(11,)) ''
  description := {
  "index": Int64Col(shape=(), dflt=0, pos=0),
  "values": Int8Col(shape=(), dflt=0, pos=1)}
  byteorder := 'little'
  chunkshape := (7281,)
  autoindex := True
  colindexes := {
    "index": Index(6, medium, shuffle, zlib(1)).is_csi=False,
    "values": Index(6, medium, shuffle, zlib(1)).is_csi=False}
  /s/table._v_attrs (AttributeSet), 11 attributes:  [CLASS := 'TABLE',
    FIELD_0_FILL := 0,
    FIELD_0_NAME := 'index',
    FIELD_1_FILL := 0,
    FIELD_1_NAME := 'values',
    NROWS := 11,
    TITLE := '',
    VERSION := '2.7',
    index_kind := 'integer',
    values_dtype := 'category',
    values_kind := ['values']]
  Data dump:
[0] (0, 0)
[1] (1, 0)
[2] (2, 1)
[3] (3, 1)
[4] (4, 2)
[5] (5, 3)
[6] (6, 4)
[7] (7, 3)
[8] (8, 5)
[9] (9, 0)
[10] (10, 1)
/s/meta (Group) ''
  /s/meta._v_attrs (AttributeSet), 3 attributes:
   [CLASS := 'GROUP',
    TITLE := '',
    VERSION := '1.0']
/s/meta/values (Array(6,)) ''
  atom := StringAtom(itemsize=1, shape=(), dflt='')
  maindim := 0
  flavor := 'numpy'
  byteorder := 'irrelevant'
  chunkshape := None
  /s/meta/values._v_attrs (AttributeSet), 5 attributes:
   [CLASS := 'ARRAY',
    FLAVOR := 'numpy',
    TITLE := '',
    VERSION := '2.4',
    kind := 'string']
  Data dump:
[0] a
[1] b
[2] c
[3] d
[4] e
[5] f
```
 That change makes sense.  And with compression large chunks of whitespace might be less of an issue anyway.
 for example. Its actually a function of the max_length of the strings stored. 

```
In [27]: df = DataFrame({'A' : np.random.randn(5), 'B' : Series(['a','foo','bar','a really long string','baz'])})

In [28]: df_cat = df.copy()

In [29]: df_cat['B'] = df_cat['B'].astype('category')

In [30]: pd.concat([df]*10000).to_hdf('test1.h5','df',mode='w',format='table')

In [31]: pd.concat([df_cat]*10000).to_hdf('test_cat.h5','df',mode='w',format='table')

In [33]: !ls -ltr *.h5
-rw-rw-r--  1 jreback  staff  1876493 Nov 14 10:02 test1.h5
-rw-rw-r--  1 jreback  staff   895756 Nov 14 10:02 test_cat.h5
```
 @jorisvandenbossche ?
 Is this a format change? What will happen if someone wants to read with an older version of pandas an hdf file that is saved with 0.15.2 (or was such a thing never supported?)
 Ok, I updated to make this more explicit. It is now backwards AND forwards compatible. In that you can read a >0.15.2 written file in a prior version.

You will get the codes in the table (as that is how they are stored).
The categories are now stored as a regular pathed array, so they can also be retrieved.
So it loss-less in a forward way (but requires the user to use them, as the `Categorical` type did not exist prior to 0.15.0.)

The following in 0.15.2

```
In [1]:    dfc = DataFrame({ 'A' : Series(list('aabbcdba')).astype('category'),
   ...:                      'B' : np.random.randn(8) })

In [2]:    store = pd.HDFStore('test.h5', mode='w')

In [3]:    store.append('df', dfc, format='table', data_columns=['A'])

In [4]:    result = store.select('df', where="A in ['b','c']")

In [5]:    result
Out[5]: 
   A         B
2  b  0.259910
3  b -0.489301
4  c -1.681019
6  b -2.147062

In [6]:    result.dtypes
Out[6]: 
A    category
B     float64
dtype: object

In [7]: store
Out[7]: 
<class 'pandas.io.pytables.HDFStore'>
File path: test.h5
/df                        frame_table  (typ->appendable,nrows->8,ncols->2,indexers->[index],dc->[A])
/df/meta/A/meta            series       (shape->[1])                                                 

In [8]: store.select('df/meta/A/meta')
Out[8]: 
0    a
1    b
2    c
3    d
dtype: object
```

and in 0.15.1 reading the same file

```
In [1]: store = pd.HDFStore('pandas/test.h5')

In [2]: store
Out[2]: 
<class 'pandas.io.pytables.HDFStore'>
File path: pandas/test.h5
/df                        frame_table  (typ->appendable,nrows->8,ncols->2,indexers->[index],dc->[A])
/df/meta/A/meta            series       (shape->[1])                                                 

In [3]: store.select('df')
Out[3]: 
   A         B
0  0 -0.906125
1  0  1.324821
2  1  0.259910
3  1 -0.489301
4  2 -1.681019
5  3  0.711411
6  1 -2.147062
7  0  0.797939

In [4]: store.select('df').dtypes
Out[4]: 
A       int8
B    float64
dtype: object

In [5]: store.select('df/meta/A/meta')
Out[5]: 
0    a
1    b
2    c
3    d
dtype: object
```
 You know added some docs to categorical.rst, but maybe also add (or refer to) something in io.rst#pytables ?
 hmm ok sure
 ok, fixed up
 @jorisvandenbossche any further comments?
 nope, no further comments! (but for the actual pytables interaction, I am not familiar with that)
