WIP - Use broccoli-merge-trees

This would fix https://github.com/stefanpenner/ember-cli/issues/265

It's not ready yet because it breaks on https://github.com/HeroicEric/ember-cli/blob/broccoli-merge-trees/lib/preprocessors.js#L24-L38. I believe it's because this isn't returning a broccoli tree.

The error that is returned when attempting to precompileCss:

```no-highlight
Object [object Object],[object Object],vendor,[object Tree:vendor/ember],[object Tree:vendor/ember-data],[object Tree:vendor/ember-qunit],[object Tree:vendor/ember-resolver],[object Tree:vendor/handlebars],[object Tree:vendor/ic-ajax],[object Tree:vendor/jquery],[object Tree:vendor/loader],[object Tree:vendor/qunit],[object Tree:vendor],[object Tree:vendor],[object Tree:vendor] has no method 'read' TypeError: Object [object Object],[object Object],vendor,[object Tree:vendor/ember],[object Tree:vendor/ember-data],[object Tree:vendor/ember-qunit],[object Tree:vendor/ember-resolver],[object Tree:vendor/handlebars],[object Tree:vendor/ic-ajax],[object Tree:vendor/jquery],[object Tree:vendor/loader],[object Tree:vendor/qunit],[object Tree:vendor],[object Tree:vendor],[object Tree:vendor] has no method 'read'
    at readTree (/Users/eric/Dropbox/sandbox/ember/ember-cli/node_modules/broccoli/lib/builder.js:61:20)
    at StaticCompiler.Transform.read (/Users/eric/Dropbox/sandbox/ember/thang/node_modules/broccoli-static-compiler/node_modules/broccoli-transform/index.js:13:10)
    at readTree (/Users/eric/Dropbox/sandbox/ember/ember-cli/node_modules/broccoli/lib/builder.js:61:20)
    at promise.then.then.results.(anonymous function) (/Users/eric/Dropbox/sandbox/ember/thang/node_modules/broccoli-merge-trees/node_modules/promise-map-series/index.js:8:27)
    at invokeCallback (/Users/eric/Dropbox/sandbox/ember/thang/node_modules/broccoli-merge-trees/node_modules/promise-map-series/node_modules/rsvp/dist/commonjs/rsvp/promise.js:228:21)
    at publish (/Users/eric/Dropbox/sandbox/ember/thang/node_modules/broccoli-merge-trees/node_modules/promise-map-series/node_modules/rsvp/dist/commonjs/rsvp/promise.js:176:9)
    at publishFulfillment (/Users/eric/Dropbox/sandbox/ember/thang/node_modules/broccoli-merge-trees/node_modules/promise-map-series/node_modules/rsvp/dist/commonjs/rsvp/promise.js:312:5)
    at flush (/Users/eric/Dropbox/sandbox/ember/thang/node_modules/broccoli-merge-trees/node_modules/promise-map-series/node_modules/rsvp/dist/commonjs/rsvp/asap.js:41:9)
    at process._tickCallback (node.js:415:13)
```

I'm going to try and figure this part out now but if anybody more knowledgable on this wants to help out, let me know!