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
Return correct column type on querying. Add tests for the same
Move limit values to class constants
Merge remote-tracking branch 'twistedR/mysql-text-columns-support' into 0.3.x-dev

Conflicts:
	tests/Phinx/Db/Adapter/MysqlAdapterTest.php

**Note:** Tweaked code in `classifyTextColumn()` before merge (@shadowhand)