Throw an error is we encounter an unknown enum value while decoding (JS)

[bvandenbos]

Fixes #11 and adds some missing enum tests.

In the javascript library, while decoding, Kiwi will throw an error if it encounters an field it doesn't know about. It won't, however, throw an error if it encounters an Enum value it doesn't know about. The decoded value will simply be undefined.

[kevzettler]

Code lgtm. This PR has conflicts and needs a rebase.