API Cleanup (breaking changes!)

I talked myself into cleaning up the API to improve clarity.

Removed the following methods:

• EnumWrapper.prototype.has - Use isKey instead
• EnumWrapper.prototype.get - Use getValueOrDefault (or getValueOrThrow) instead

Renamed the following methods:

• EnumWrapper.prototype.asKey -> asKeyOrThrow
• EnumWrapper.prototype.getKey -> getKeyOrThrow
• EnumWrapper.prototype.asValue -> asValueOrThrow
• EnumWrapper.prototype.getValue -> getValueOrThrow

Read the conversation I had with myself here: https://www.reddit.com/r/typescript/comments/7ykp3j/typescript_enum_utils_strictly_typed_wrapper/duhrqq5/

Initial Semi-Stable Release

The overall design and API of this project seems to have stabilized, and the README has reached the point of being useful. Time to release this into the wild and see what kind of feedback I get.