Capybara 2.0 prelim support, document update, rake update

Updated Rakefile to support new RDoc library structure.

Added details on iterating over multiple browsers to the Readme.

Added preliminary support for Capybara ~>2.0.0.  This is a breaking change with ~>1.0.0 so will need some discussion OR versioning hackery OR maintenance of two release branches until we drop support for Capybara < 2.