Major performance boost. Fix #126

This major refactoring boosts performance by 70% - 75% for all browsers by introducing a custom `Backgrid.View` and having all Backgrid components inherit from it. `Backgrid.View` closely resembles `Backbone.View`'s semantics with the exception that it isn't backed by jQuery.

This branch has been tested successfully on all desktop browsers: IE8+, lastest versions of Firefox, Safari, Chrome and Opera. IE8 support will be downgraded and a `classList` shim is required now. All tests still pass for IE8 however once a `classList` polyfill is in place.

TODO:
- jashkenas/backbone#2959 is blocking this merge. Specially, whether `Backgrid.View` will stay or go, and how much shimming is required for IE8 depends on whether the PR gets merged.

Interested parties please help review this code.