Add Playground#request_filter for AbTest#choose(s).

* Closes #175
* Will filter based in JS and non-JS experiments
* Fix default value of Playground#on_datastore_error

Conflicts:
	lib/vanity/experiment/ab_test.rb
	lib/vanity/frameworks/rails.rb
	lib/vanity/playground.rb
Merge pull request #176 from assaf/feature/block-bots

Add Playground#request_filter for AbTest#choose(s).