Add support for TINY/MEDIUM/LONGTEXT columns in MySQL

The column type is now selected based on the limit specified. This is done the Rails way. The limit must be specified in bytes. The limits are selected based on documentation here
http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html

If this is okay I can add support for TINYINT as well.
Again, that's totally fine, but a constant (eg: `Migration::TEXT_SMALL = 124` and `TEXT_MEDIUM = 65536`) would do a lot for usability.