Remove existing codemirror package.

The contents of third_party/codemirror was the result of a direct git
checkout of the codemirror git repository. There are a lot of misc files
we won't be needing there, so let's start fresh by removing all the
files.
Add Bower support for codemirror.dart

Our bower.json relies on 4.4.0, same version as before.

Important files:

bower.json: skeletal Bower manifest file.
.bowerrc:   ensures dependencies install to third_party/
Use a Bowerified version of CodeMirror.

Update grind.dart to point to the new asset paths.
Add simple instructions on how to use Bower.
Merge pull request #2 from radicaled/bower_power

Bower power