Disallow empty multiple choice constraint arrays.
Removed unused msgpack requirement from ClassyHash test spec.
Implemented constraint generators in ClassyHash::Generate:

Implemented length, string length, array length, and enum generators.
Wrote extensive tests for all generators.
Added generator examples to README.
Bumped version number accordingly.
Added SimpleCov for test coverage tracking.
Added tests to hit missed lines as reported by SimpleCov:

Tested error message generation for array within multiple choice.
Tested invalid length limits in length generators.
Tested using :optional as a constraint.