Added support for tinyint in MySQL adapter

Hello, 

I needed to use tinyint, so I've added the column type to the MySQL adapter.

To use tinyint column type, use 'tinyinteger' as type value when
creating or updating a column.

Eg. $this->add_column('table', 'column', 'tinyinteger');

Postgresql has no equivalent for tinyint, so falls back to smallint
type when 'tinyinteger' is used.

MySQL 5.0 documentation for the tinyint column type can be found at
http://dev.mysql.com/doc/refman/5.0/en/integer-types.html

Thanks, 
SB