Re-write Core Functionality to support all markups Tilt Supports, Template chaining

- If Tilt supports it, ART supports it
- Templates can be chained (e.g.: html.haml.erb)
- common markups are autoloaded, additional markups can be loaded by adding them to config.angular_templates.markups
-  If the user has the required library loaded the following formats/extensions will be added to sprockets engines: [erb haml liquid md radius slim str textile wiki]
- Use the JSON gem for its consistent json-encoding behavior

Minor:
- Use #sub instead of #gsub
- remove the newline at the end of the html (if present)
Major Testing Improvements (Minitest, Capybara, dozens of new tests)

- remove CSS stylesheet - it was being compiled, and we don't care about it
- don't generate digested manifest - no need to compile assets twice
- support MinitTest 4 and 5 - don't use Test::Unit
- Use Capybara for integration tests
- Test functionality in integration tests, not in "the precompile" test.
- use both secret_token/secret_key_base for Rails 3/4 compatibility
- Run trivial ruby in the .erb and .str templates
- Test the inclusion of templates from app/assets/templates
- Have travis test against Rails head
- test against ~> 4.0.X, ~> 3.2.X instead of a fixed version
- Add kramdown to test markdown`
Add AngularJS 1.2.16 and a trivial app to the Dummy App

- angular is not in the manifest, but in dummy/public
- trivial angular app shows how many templates are in the cache, angular version number
- `rackup` in test/dummy to see the demo app