Merge pull request #3 from kpumuk/master

Multiple enhancements to the project
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Features:

  - Added "enum_value" (returns value) instance method in addition to "enum_name" (returns stringified enum key)
  - Added class methods "enum_value" (returns value by key) and "enum_by_value" (returns key by value)
  - Performance improvement: use `class_eval` with string instead of blocks

Bugfixes:

  - Fixed const naming: use SIMPLE_STATUSES and SIMPLE_STATUSES_INVERTED instead of SimpleStatuses and SimpleStatusesInverted
Preparing for version 1.0.0.beta1 release
Added Travis CI status to README