Clean up specs

Use class constants instead of strings in specs.
Rename Github to GitHub

Github just looks so weird.
Simplify requiring spec_helper
No need to push lib to paths

This is already handled by rubygems.
Restructure files

Gems that contain dashes in the name are structured as folder hierarchies.
This is the way bundler sets up a new gem.
Simplify requires in spec_helper

- warden/github is already loaded by bundler
- spec is prepended to the load path
Tame Rack::Lint

Specify a content-type for the redirect. Otherwise, Rack::Lint complains.
Remove vim magic comments
Change how debugger is loaded

- specify 1.9.3 compatible debugger gem
- don't require them through Bundler
- require the correct one in config.ru
Remove superfluous Bundler.require calls
Simplify require in config.ru
Move example app to example/
Remove unused error class
Ignore development files
Update Readme

- use relative url for warden link
- callback setting for Oauth app only needs to match host
- simplify example (everybody has bundler nowadays)