Simplify naming and make changes to get it working with wordpress-oauth 2.0

I've simplified the naming and class structure a little bit so it can be loaded as just 'omniauth-wordpress'. You also don't need to specify a 'strategy_class' this way. There are also some small changes I had to make to get it working with the newer version of the wordpress-oauth plugin.

I'm building on Sinatra and haven't had time to test this with Rails/Devise.