ENH: support for msgpack serialization/deserialization
warning: prototype!

msgpack serialization/deseriiization
- support all pandas objects: Timestamp,Period,all index types,Series,DataFrame,Panel,Sparse suite
- docs included (in io.rst)
- iterator support
- top-level api support

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

storage example `DataFrame(np.random.rand(1000,10))` on my machine stores in 128k file size.
and scales pretty well, e.g. 10k rows is 1.26mb

Not completely happy with the `packers` name, any suggestions?

closes #686

```
In [1]: df = DataFrame(randn(10,2),
   ...:                      columns=list('AB'),
   ...:                      index=date_range('20130101',periods=10))

In [2]: pd.to_msgpack('foo.msg',df)

In [3]: pd.read_msgpack('foo.msg')
Out[3]: 
                   A         B
2013-01-01  0.676700 -1.702599
2013-01-02 -0.070164 -1.368716
2013-01-03 -0.877145 -1.427964
2013-01-04 -0.295715 -0.176954
2013-01-05  0.566986  0.588918
2013-01-06 -0.307070  1.541773
2013-01-07  1.302388  0.689701
2013-01-08  0.165292  0.273496
2013-01-09 -3.492113 -1.178075
2013-01-10 -1.069521  0.848614
```
