Rename fetch render.

It was always confusing as to which `render` method you were actually modifying.  I've made it significantly clearer with this PR.  Now it is `fetchTemplate` and `renderTemplate`.  I also broke out `_render` and put it on the `Backbone.Layout.prototype`.

I also updated all the unit tests to reflect this change.

What do you guys think? Should I rename `fetch` as well? This feels like a consistent change, but also an annoying one to upgrade.