Add option to define his own cache_engine

Previously, you can cache your view render by rabl only by Rails.cache.
Now you can define you own cache_engine in Rabl::Configuration.

The Engine you can define is mandatory a class to define the #fetch
method.

By default a cache_engine exist in Rabl : Rabl::CacheEngine to see what
args take the fetch method. This method need return the result.
added README note regarding RSpec render_views
add support for Rails 3.2+ sending custom mime types
[#285] add reference to reusing templates and include example
[#283] clarify syntax for attributes in README
Merge pull request #261 from shingara/feature/define_cache_engine

Add option to define his own cache_engine
Updates changelog and prepare for 0.7.0 release
Bump version to 0.7.0
Merge branch 'master' of git://github.com/nesquena/rabl into mergey

Conflicts:
	lib/rabl/version.rb