Cleanup and Rake task to automate updating backbone.marionette.js

Did a few things:

* Fixed links to upstream in README
* Switched to a dedicated ```version.rb``` file
* Added rake task to display latest upstream tag
* Added rake task to download latest upstream tag, update vendored js, and update version.rb

It only looks at the latest tag, not all of them.

```rake -T```

    rake build              # Build marionette-rails-1.0.0.rc5.gem into the pkg directory
    rake install            # Build and install marionette-rails-1.0.0.rc5.gem into system gems
    rake marionette:latest  # Fetches and displays the latest backbone.marionette tag
    rake marionette:update  # Updates the vendored backbone.marionette version to the latest tag
    rake release            # Create tag v1.0.0.rc5 and build and push marionette-rails-1.0.0.rc5.gem to Rubygems