Add support for unsigned numeric fields in MySQLDiff backend.

Query the information_schema.columns table to discover if an numeric
field is unsigned. Use that when returning the db_type. Add
AUTO_INCREMENT check to after the unsigned signed check so db fields
that are unsigned primary keys properly conflict with AutoField.

Fixes #544
Reworked unsigned support such that it can be used by other backends.

Not implemented using the same system as NOT NULL support.
Merge pull request #545 from jdufresne/mysql-unsigned-support

Add support for unsigned numeric fields in MySQLDiff backend.