Rails 2.3 compatibility

This is for @nateberkopec. I rebased against the current master.

Stuff to do: 

* Documentation
* Write tests
* Check that the Rack middleware actually works with Rails 2.3 (I haven't tested it at all)

I did also some stylistic changes around `lib/raven/event.rb`. Mostly took out explicit block arguments where they weren't needed and replaced them with `yield foo if block_given?`, as it is a lot faster.

Would you like me to squash the commits?