Add extra docs for adding and multiplying distributions.
Add mixture_distribution().
Fix problem with mixture_distribution().
Merge branch 'master' into mixtures
Fix bug where order of sample space was changed even if sort=False.
Add is_null() to Operations.
Add option to trim sample space during construction of distributions.
Improve mixture_distribution(). Also provide mixture_distribution2().

The latter is an optimized version when all dists have the same pmf.
Add dit.utils.OrderedDict for Python 2.6 compatibility.

This points to collections.OrderedDict for Python > 2.6.
Additionally, pip will install `ordereddict` in Python 2.6.
Removing print statement used for debugging.
Add merge=False to mixture_distribution().
This allows mixture distributions between incompatible sample spaces.
Merge pull request #25 from chebee7i/mixtures

Create mixture distributions