Remove Rakefile in favor of npm scripts

It'd be great to finally break free of Backbone's one remaining ruby dependency, if for no other reason than to get all of the tooling on the same page. Backbone's Rakefile is a lot less logic-heavy since #1685, and these days it's merely a wrapper around a few system commands.

This pull doesn't change the semantics or the underlying action, just the invocation. Instead of `rake lint`, you'd run `npm run lint`, etc. If things get more complicated (need to pass options to lint, create a watch task, etc.), it's easy enough to switch to Grunt or npm tasks later.