[FEATURE] Added support for tinytext type to MySQL adapter

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