msgpack for sparse dataframes
#### Code Sample

```python
from pandas import SparseDataFrame
dfs = SparseDataFrame([[0, 0], [0, 1]])
import packers
packers.to_msgpack(None, dfs, compress='zlib')
# Returns: NotImplementedError: msgpack sparse frame is not implemented
```

#### Problem description

msgpack is much faster than pickle, which makes it highly relevant for sparse dataframes. Even "experimental" code is relevant. Speed in Python is often more relevant in experiments than in production, because code is run more frequently in a trial and errors process.

There is commented code in packers to encode / decode sparse dataframes.  What is the problem with it? What is the plan? What code snippet could we use? What alternatives do we have?

Problem is present in Master

Many thanks
