ENH: Add sparse Accessor for SparseDtype data

[TomAugspurger]

This is a blocker for removing SparseDataFrame and SparseSeries.

We'd register it for Series and DataFrame I think.

In [4]: [x for x in set(dir(pd.SparseSeries)) - set(dir(pd.Series)) if not x.startswith('_')]
Out[4]:
['from_coo',
 'block',
 'npoints',
 'sparse_reindex',
 'kind',
 'density',
 'as_sparse_array',
 'fill_value',
 'to_coo',
 'sp_index',
 'sp_values']

In [15]: [x for x in set(dir(pd.DataFrame)) - set(dir(pd.DataFrame)) if not x.startswith('_')]
Out[15]: []

[TomAugspurger]

I've started on this.