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.
Taking inspiration from Rails you should only need to specify what limit you expect. You shouldn't need to remember what mediumtext is. You should specify what length you need and library takes care of it. So let's say you have a column and the maximum information in that will be 15kb. That;s what you specify as your limit (15*1024) and it's up to phinx to choose medium or regular text.