Proposed implementation for Enum Field

[jaesivsm]

Related to #79

what I haven't done

• IntEnum
• assertions on Enum values from typed Enum (only strings, only integers)
• documentation, I didn't find the place where every type are explained with and example. If there's one I'd gladly add documentation about that field.

[j5awry]

Using the ticket for a higher level discussion of enum implementation. Please toggle back to #79 to see general comments.

Also be sure to check the CONTRIBUTING.md. We'd expect tests to be added for the addition.