Improvements to Rackspace Cloud Files action. New action for Slack

I made several changes to the Rackspace Cloud Files action:
* switched from camelCase to snake_case
* could not reproduce the Exception you got, but added a rescue, please verify that is handled
* handling a RuntimeError on creating container which happened for me when trying to connect to London server.
* added support for custom domain

I added a new action for Slack:
* it allows you to upload files into a channel from Slack
* initially I did it for Ruby 2.0, switched it to 1.8 to increase compatibility
* initially I implemented also Text support, but then I realised that the Slack API doesn't allow posting messages as yourself through the API, only as a bot (confirmed by them on Twitter). Therefore I disabled that feature for the moment.
* although I proceeded as with the Rackspace action, I needed to change some of the faraday classes to find the multipost library, any clues why?