msgpack unpack dataframe error: InvalidIndexError
This happen with some Pandas dataframes with indexes. 

``` python
from pandas import read_msgpack
msg = df.to_msgpack()
read_msgpack(msg) #==> exception!
```

Full Traceback (most recent call last):

File "/srv/python/venv/local/lib/python2.7/site-packages/pandas/io/packers.py", line 163, in read_msgpack
return read(fh)
File "/srv/python/venv/local/lib/python2.7/site-packages/pandas/io/packers.py", line 141, in read
l = list(unpack(fh))
File "pandas/msgpack.pyx", line 662, in pandas.msgpack.Unpacker.**next** (pandas/msgpack.cpp:8303)
File "pandas/msgpack.pyx", line 591, in pandas.msgpack.Unpacker._unpack (pandas/msgpack.cpp:7328)
File "/srv/python/venv/local/lib/python2.7/site-packages/pandas/io/packers.py", line 490, in decode
blocks = [create_block(b) for b in obj['blocks']]
File "/srv/python/venv/local/lib/python2.7/site-packages/pandas/io/packers.py", line 488, in create_block
placement=axes[0].get_indexer(b['items']))
File "/srv/python/venv/local/lib/python2.7/site-packages/pandas/core/index.py", line 1488, in get_indexer
raise InvalidIndexError('Reindexing only valid with uniquely'
InvalidIndexError: Reindexing only valid with uniquely valued Index objects
