Move Javascript to the asset pipeline

* Move all Diaspora-specific javascripts to app/assets/javascripts
* Move all vendored javascripts to vendor/assets/javascripts
* Add the appropriate Sprockets require directives to make sure
  everything gets included in the right order
* Remove Jammit dependencies
* Fix all templates that were using Jammit's include_javascripts helper
* Add handlebars_assets gem for compiling Handlebars templates
* Move all Handlebars templates to app/assets/templates and rename
  from .handlebars to .jst.hbs (this is to keep them in the same
  global JST namespace that they were in under Jammit)
* Add public/assets to .gitignore since these files can and should
  be re-generated by Heroku or Capistrano during each deploy
* Fix a few Handlebars templates that were looking for images in the
  wrong location (I'm sure there are others, but it's late)
* Configure application.rb to precompile all javascript and css assets
  that were compiled by Jammit in the Rails 3.0 code