Ndd

Added
- added `ndd` to the anaconda `conda-forge` channel.

Fixed
- fixed `setuptools` imports in `setup.py`

Changed
- `entropy` function: removed the `zk` argument
- the signature of the `entropy` function is:

entropy(nk, k=None, estimator='NSB', return_std=False)

Added
- the `entropy` function handles mappings and generators as input counts.

Fixed
- fix Counts.fit()

Changed
- if argument `k` is omitted, the `entropy` function will guess a reasonable
alphabet size and select the best estimator for the sampling regime.