Implement ENUM data type

[bgrainger]

Implement support for ENUM.

http://dev.mysql.com/doc/refman/5.7/en/enum.html

Required for #4 and PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#36

[caleblloyd]

Does Connector/NET support SET or ENUM? I can't seem to find it. Let's keep the conversation here even though #71 is related.

[bgrainger]

I don't think it has specialised .NET types to wrap them, but does support deserializing them (both to string I think). I was just looking for the same level of basic support in this case.

[bgrainger]

ENUM column values are returned as strings by MySQL Server and will be exposed as strings by MySqlDataReader.GetValue.