Set charset/collation properly for each text column if using MySQL.

With this change, Huginn is able to store up to 4-byte UTF-8 characters
in its database.  This should fix #286.
Add support for Ruby 1.9.x.
Fall back utf8mb4 to utf8 if the server does not support it.
Follow the default collation of Rails rather than MySQL.

`utf8*_unicode_ci` is known to be problematic in some languages like
Japanese, but hopefully it wouldn't do harm in practice in our case.