BUG: read_pickle and period data
I can pickle dataframes with periods but after import Period format becomes corrupted. After pickle Period stride appears to be none instead of 'Q' in the following example. 

<pre><code>
import pandas as pd

df = pd.DataFrame({'period': [pd.Period('2014Q4')] * 5})
msk = df['period'] == pd.Period('2014Q4')
print msk

df.to_pickle('tmp.pkl')
df = pd.read_pickle('tmp.pkl')

msk = df['period'] == pd.Period('2014Q4')
</code></pre>


<pre><code>
Traceback (most recent call last):
  File "/Users/scarrucciu/Downloads/period_pickle_test.py", line 11, in <module>
    msk = df['period'] == pd.Period('2014Q4')
  File "/Users/scarrucciu/anaconda/lib/python2.7/site-packages/pandas/core/ops.py", line 614, in wrapper
    res = na_op(values, other)
  File "/Users/scarrucciu/anaconda/lib/python2.7/site-packages/pandas/core/ops.py", line 562, in na_op
    result = lib.scalar_compare(x, y, op)
  File "pandas/lib.pyx", line 696, in pandas.lib.scalar_compare (pandas/lib.c:12434)
  File "das/src/period.pyx", line 743, in pandas._period.Period.__richcmp__ (pandas/src/period.c:11121)
ValueError: Cannot compare non-conforming periods
</code></pre>

## INSTALLED VERSIONS

commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Darwin
OS-release: 13.4.0
machine: x86_64
processor: i386
byteorder: little
pandas: 0.16.2
