Implement SET data type

[bgrainger]

Implement support for SET.

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

Required for #4.

[bgrainger]

MySQL allows a column type to have a type of SET (see the documentation linked above).

This library needs to allow clients to query columns of that data type, insert new data, etc. It should expose the same .NET types as MySql.Data does, for compatibility with that library.

At a minimum, new tests that use this data type need to be added to test the level of support that currently exists and show what's missing.