MySQL support

This commit adds MySQL v5.6.4+ support to Thredded.

* MySQL Full Text Search via `MATCH AGAINST` and a `:fulltext` index.
* Migrations compatible with both MySQL and PostgreSQL
** MySQL-only `limit: 191` on indexed string fields (slug, title)
  http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html
* Both of the schemas get dumped, `db:schema:load` loads the active one
  as per `ENV['DB']`.
* Tests run and pass on both PostgreSQL and MySQL.
* Test gemfiles for Rails 4.0 and 4.1.
* Remove Gemfile.lock from version control, upgrade to RSpec 3, specify
  versions of bbcoder, rspec-rails, capybara, and shoulda-matchers in
  gemspec.
* Fix compatibility issues in `messageboard_destroyer.rb`.
* Fix MySQL latest_post/user timestamp ordering issue in
  `messageboard_decorator.rb`.
* Fix compatibility issues in `Topic`. Extract shared code from `Topic`
  and `PrivateTopic` into the `TopicCommon` concern.