Make it work with Sinatra

When we require "sinatra/jbuilder", Sinatra can use `jbuilder` method to
handle `*.jbuilder` templates.

To store an instance of Sinatra application inside a `Jbulder`
instance, `::Tilt::Jbuilder` was subclassed from `::Jbuilder`.
Render partials with helper methods, instance variables, and local variables.

Pass an instance of Sinatra application to Tilt::Jbuilder for partials.
Merge pull request #2 from a2ikm/sinatra-integration

Make it work with Sinatra