Added support for mediumint in MySQL adapter

Adds support for missing mediumint column type in MySQL adapter.

To use mediumint column type, use 'mediuminteger' as type value when
creating or updating a column.

Eg. $this->add_column('table', 'column', 'mediuminteger');

Postgresql has no equivalent for mediumint, so falls back to integer
type when 'mediuminteger' is used.

MySQL 5.0 documentation for the mediumint column type can be found at
http://dev.mysql.com/doc/refman/5.0/en/integer-types.html