Implement __contains__ ('x' in Choices('x')) for Choices objects.

In Choices, `_choice_dict` appears to be a definitive location of all
internal/DB representations, so it seems the best target for finding
out if the given item is part of the sequences.
first pass at tests for the __contains__ functionality
just implemented.
Merge pull request #69 from kezabelle/feature/choices_contains

Implementing __contains__ in Choices