Support for Rails 3 and current Uservoice API

[lacco]

Changes:

• Make generator and tests work in Rails 3
• Adjust config and SSO to current version of Uservoice API (you now define UI-based settings of widgets on the Uservoice side and not in the app anymore)
• Tests are using a Rails engine instead of defining controllers inline
• Updated README (using markdown instead of rdoc)
• Using a Gemfile for dependencies
• Config can be defined in config/uservoice.yml or via ```Uservoice.config.merge!(...)