Ch5.Ex1. Models that can be updated

+ now using Rack::Request to parse the request, get parameters, etc. (Rails does this too)
+ implemented a RESTful-like update URL pattern that delegates to the controller's "create" when client sends HTTP POST.
+ FileModel now can save its contents to the original file.
(refactor) Inline extraneous local variables to improve readability

+ using MultiJson to render out to JSON instead of doing it by hand.
+ where it doesn't hurt readability, inlined local variables.
Merge branch 'master' of github.com:jtigger/ruby-on-rulers

Conflicts:
	lib/rulers/file_model.rb