BUG: fix PeriodConverter issue when given a list of integers (GH9012)
For example coming up when [0, 1] is passed by axhline

Closes #9012 

test and whatsnew are coming

the perf diff was massive
you need to find a better way 
 something like this

```
In [8]: from pandas.tseries.converter import get_datevalue

In [9]: pr = pd.period_range('2000',periods=10000,freq='D')

In [10]: [ get_datevalue(v,pr.freq) for v in pr ][0:10]
Out[10]: [10957, 10958, 10959, 10960, 10961, 10962, 10963, 10964, 10965, 10966]

In [11]: pd.PeriodIndex(pr).values
Out[11]: array([10957, 10958, 10959, ..., 20954, 20955, 20956])

In [12]: %timeit pd.PeriodIndex(pr).values 
100000 loops, best of 3: 3.96 us per loop

In [13]: %timeit [ get_datevalue(v,pr.freq) for v in pr ]      
10 loops, best of 3: 116 ms per loop
```

Though i don't think this is actually causing the bug, they return the same results AFAICT
 In this case they return the same (as it are periods), but not when you provide eg ints:

```
In [2]: values = [0, 1]

In [3]: [ get_datevalue(v, 'B') for v in values ]
Out[3]: [0, 1]

In [4]: pd.PeriodIndex(values, freq='D').values
...
DateParseError: day is out of range for month
```
 And the thing is, if it is a PeriodIndex, it is already special cased: https://github.com/pydata/pandas/blob/v0.15.1/pandas/tseries/converter.py#L116, so this will not impact perfomance. 

But, I think if you just do `ts.plot()` the x values are not provided as a PeriodIndex, but as an ndarray of Periods, so that hits the line in question, and in this way it will indeed impact performance.
 ok, so obviously need a test for that, I thing should be straightforward to fix. The thing is to avoid reinterpreting the freq when you already have a PeriodIndex.
 Maybe specifically check for an array of Periods?

```
In [38]: pr = pd.period_range('2000',periods=10000,freq='D')

In [39]: a = np.array([p for p in pr])

In [40]: a
Out[40]:
array([Period('2000-01-01', 'D'), Period('2000-01-02', 'D'),
       Period('2000-01-03', 'D'), ..., Period('2027-05-16', 'D'),
       Period('2027-05-17', 'D'), Period('2027-05-18', 'D')], dtype=object)

In [41]: pd.lib.infer_dtype(a)
Out[41]: 'period'

In [42]: pd.core.common.is_period_arraylike(a)
Out[42]: True
```

This is the performance difference (that you fixed for the general time series plotting):

```
In [49]: pd.PeriodIndex(a, freq='D').values
Out[49]: array([10957, 10958, 10959, ..., 20954, 20955, 20956], dtype=int64)

In [50]: [get_datevalue(x, 'D') for x in a][0:5]
Out[50]: [10957L, 10958L, 10959L, 10960L, 10961L]

In [51]: %timeit pd.PeriodIndex(a, freq='D').values
100 loops, best of 3: 7.88 ms per loop

In [52]: %timeit [get_datevalue(x, 'D') for x in a]
10 loops, best of 3: 167 ms per loop

In [53]: s = pd.Series(np.random.randn(10000), index=pd.date_range('2012-01-01', periods=10000, freq='s'))

# current master
In [36]: %%timeit
   ....: fig, ax = plt.subplots()
   ....: s.plot(ax=ax)

1 loops, best of 3: 207 ms per loop

# patched with how it was before
In [38]: %%timeit
   ....: fig, ax = plt.subplots()
   ....: s.plot(ax=ax)

1 loops, best of 3: 353 ms per loop

```
 you prob can use `isinstance(a, np.ndarray) and is_period_arraylike(a)` 

because then a PeriodIndex would be excluded
 yes, but I don't think that is needed, because: PeriodIndex is already checked for in an `if` before that line, and also lists of Periods can go directly to PeriodIndex(..) I think
 ok then!
 ah, but `is_period_arraylike` does not seem to handle lists of Periods (gives False)
 ```
In [1]: pr = pd.period_range('20130101',periods=3,freq='D')

In [2]: pr.tolist()
Out[2]: 
[Period('2013-01-01', 'D'),
 Period('2013-01-02', 'D'),
 Period('2013-01-03', 'D')]

In [3]: np.array(pr.tolist())
Out[3]: 
array([Period('2013-01-01', 'D'), Period('2013-01-02', 'D'),
       Period('2013-01-03', 'D')], dtype=object)

In [4]: pandas.core.common.is_period_arraylike(np.array(pr.tolist()))
Out[4]: True
```

really
 yeah, but I expected it to handle lists without having to convert it to array:

```
In [67]: pandas.core.common.is_period_arraylike(pr.tolist())
Out[67]: False
```

But in any case, not going to worry about that particular case for now
 ahh, ok, that makes sense then (it assuem ndarray ness)
 OK, updated the PR, no time now to further look at it, but will try to add tests later this evening
 ok, will try to do the release tonight in that case 
 merged via https://github.com/pydata/pandas/commit/b69b9d3e208c0a0ac6e1f26ae1fb4ec10d0cd7bb
