ENH: support for msgpack serialization/deserialization

warning: prototype!

msgpack serialization/deseriliization
  - support most pandas objects: Timestamp,Period,all index types,Series,DataFrame
  - missing Sparse, and Panel (not too hard to add), and docs

Here are 2 features which I think msgpack supports, but have to look further
  - no support for compression directly, but can compress the file (e.g. gzip)
  - access is sequential
  - versioning, is not that hard because its pretty easy to deal with a 
     change in the schema (which is not directly stored), and this is MUCH more
     transparent than pickle

usage is exactly like pickle (aside from that its in a different namespace), allowing
arbitrary combinations of storage, e.g. this supports the added storage of pandas objects,
but obviously can store ``{ 'frame1' : df1, 'frame2' : df2 }`` etc

Not completely happy with the ``packers`` name, any suggestions?

closes #686

```
In [2]: df = DataFrame(randn(10,2),
                     columns=list('AB'),
                     index=date_range('20130101',periods=10))

In [6]: from pandas.io import packers

In [7]: packers.save('foo.msg',df)

In [8]: packers.load('foo.msg')
Out[8]: 
                   A         B
2013-01-01 -1.222067  0.909417
2013-01-02 -1.452324  0.121088
2013-01-03  1.291239 -2.379701
2013-01-04  0.018053  0.522068
2013-01-05 -1.560006  1.230141
2013-01-06  0.774055  0.484828
2013-01-07  0.996267 -0.664828
2013-01-08  2.586835  1.176232
2013-01-09  0.578000  0.296687
2013-01-10  0.064407  0.992416
```