Added several improvements to the structure

* added some useful rake tasks:

```ruby
rake build     # Build omniauth-twitch_oauth2-0.0.1.gem into the pkg directory. / Build gem
rake bump      # Update version number and gemspec
rake console   # Open an irb session preloaded with this library
rake gemspec   # Update gemspec
rake install   # Build and install omniauth-twitch_oauth2-0.0.1.gem into system gems.
rake release   # Create tag v0.0.1 and build and push omniauth-twitch_oauth2-0.0.1.gem to Rubygems / Create a release build
rake spec      # Run RSpec code examples
rake validate  # Validate lib files and version file
```

* moved version into lib/omniauth-twitch_oauth2.rb because it is overhead to load another file.


* renamed the gemspec file, because of gem autoloading issues.

* added rubyforge specific things if you plan to release it (hope you will)

* added specs for testing

* added travis.ci support for multiplattform testing and CI