Add HTTP options for RestClient::Request

There are cases where one needs to pass additional options to the underlying HTTP connection. This can be achieved by allowing `MediaWiki::Gateway#initialize` to receive an additional argument which is then being passed down to `RestClient::Request.execute`. In order to do that, the `RestClient.{get,post}` calls need to be replaced with their `RestClient::Request.execute` equivalents.

This change should be fully backwards compatible.

(Note: I updated the specs to work with RSpec 3 -- feel free to ignore that commit.)