Fix some pprint_thing warts
**Part 1**
currently:

``` python
In [4]: from pandas.core.common import pprint_thing as pp_t
   ...: pp_t([1,None])
Out[4]: u'[1, ]'
```

after PR:

``` python
In [13]: from pandas.core.common import pprint_thing as pp_t
    ...: pp_t([1,None])
Out[13]: u'[1, None]'
```

and jeff fixed index NaN printing in #3034, at a higher level.

**Part 2**

currently:

``` python
In [1]: pd.Index(['a','b'])
   ...: 
Out[1]: Index([a, b], dtype=object)
```

after PR:

``` python
pd.Index(['a','b'])
Out[3]: Index([u'a', u'b'], dtype=object)
```

so:

``` python
In [7]: from pandas import Index
In [10]: eval(repr(pd.Index([u'a', u'b'], dtype=object)))
Out[10]: Index([u'a', u'b'], dtype=object)
```

Which is what `repr()` should enable, in principle.

added notes to 0.11.0., but maybe this should wait until 0.12/0.11.1

also, make Timstemp repr executable, like datetime.
 @y-p I only found the 1 case where a NaN was actually passed to pprint_thing (in index.py/format, which I fixed)
 yep. what bothers you about these changes?
 is there a test case where Part 1 actually occurs? 
 say there isn't. Why should None be invisible, even in theory?
 oh...nothing....they look fine...just curios (I have always though we should ALWAYS convert None anyhow...but that is a separate issue)
 I must have written that at the spur of some 4am moment, ["that's no basis for a system of government"](http://www.spunk.org/texts/humour/sp000621.txt).
 that's pretty funny!
 rebase fail, redone with 4aea3f2
