Added support for mediumint in MySQL adapter

Hello, 

I needed to use mediumint, so I've added the column type to the 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

Thanks, 
SB