Comment/docstring updates.
Add module docstring for lattice operations.

The docstring explains why/how the lattice operations depend on the
underlying sample space of the distribution.
Add is_null_exact() to ops.
Add pruned_samplespace() and expanded_samplespace().

These functions create new distributions with pruned or expanded sample
spaces.
Port distribution_from_data() from CMPy.
Automatically import dit.example_dists.
Don't import pycounts if Cython isn't available.
Clean up exceptions and do PEP8 stuff.
Add simplex_grid().
Add inplace=False to simplex_grid()
Modify history representation for counts_from_data.
standardize_data now returns the alphabet.
Make prune_expand work with log distributions.
Updated Cython generated c file.
Optimize standardize_data for alphabet size k <= 4.
Add dit.copypmf().
Use repr for msg in bad normalization exception.
Add classes for sample spaces.
Allow sorting and indexing into sample spaces.
Use a BaseSampleSpace class.