Add asset gems to Gemfile

* From advice found in https://mutelight.org/asset-pipeline
Add fixture for app with assets
Add basic root request spec for app with assets
Add actual asset stuff to assets spec + fixture
Load CSS/JS assets in app#call
Update docs for assets

* Fix typos in asset specs
Add app spec for 404ing asset
Add spec for image requests
Add TODOs to test for content types in asset specs
Make image asset specs check contents

* Split up image asset specs into 3 separate ones
Get content type working for served assets
Whitespace
Move asset path check to private class method
Add img/link/script tags to assets fixture
Merge pull request #5 from jakeboxer/assets

Get assets working