MySQL compatible migrations

Make migrations run on MySQL.

The `limit: 191` on indexed string fields is due to [a mysql limitation](http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html):

> InnoDB has a maximum index length of 767 bytes, so for utf8 or utf8mb4 columns, you can index a maximum of 255 or 191 characters, respectively. 

`utf8mb4` is required for emoji support, thus 191.

@jayroh By the way, have you seen how [Comfortable Mexican Sofa](https://github.com/comfy/comfortable-mexican-sofa) gem handles schema and migrations?