Incremental compilation

By enabling incremental compilation in Gruntfile and running `grunt server`, individual tasks will listen to file change events and when a file is modified / created / deleted, that task will be run. If a file is deleted, the destination file of that task will be first removed. Furthermore, the scope of the task will be narrowed down to the modified file. (This can be toggled on/off in task config.)

[Example output](https://gist.github.com/mbixby/6537343).
 
No verbose configuration for `grunt-contrib-watch` is necessary since the task chain is automatically constructed by emitting / receiving file change events. 

Drawbacks: no tests, out of scope of ember-appkit. 

If you pull [the feature branch](https://github.com/mbixby/ember-app-kit/tree/feature/incremental-compilation) into your existing repo, don't forget to update `grunt-contrib-watch` to 0.5.0.

Incremental compilation is disabled by default in Gruntfile and then is only enabled for `build:scripts` task.

@rjackson mentioned `grunt-newer` in #134 and it seems nicer, although I'm not sure if it handles creating files.