Autodiscover renderers rather than use hard-coded list.

Finds renderers automatically under the Rickshaw.Graph.Renderer namespace rather than using a hard-coded list within Rickshaw.Graph's initialize.

Allows third-party developers to produce new renderers without having to modify their rickshaw.js and have merge conflicts when they upgrade.

Implementation is a bit hacky, there's probably a cleaner way, but it's an improvement over the existing inaccessible hard-coded list.

Test Plan:

 * cd to your rickshaw repo
 * checkout master
 * `make build` to get a version of rickshaw.js without the patch applied.
 * check out the patch branch, making sure you carry over the master version of rickshaw.js
 * `npm test` and verify that the new unit test fails on the unpatched rickshaw.js
 * `make build` to get a version of rickshaw.js with the patch applied.
 * `npm test` and verify that the new unit test passes on the patched rickshaw.js.

Test plan has been tried and passes on my local repo.