Browser fixes and Karma test runner

/to @bajtos 
/cc @raymondfeng 

> Note: this is based on https://github.com/strongloop/loopback/pull/172
> It is only the basic browser support, the server connector and other changes will come in separate PRs.

**Bundling** 

The browser distribution is bundled using browserify. This is done in a couple of places in the gruntfile. Would be nice to only have a single way to bundle. Suggestions welcome for how to approach this.

**Testing** 

Refactored the test runner to just use karma. The tests are passing on Chrome at the moment. Need to install some other browsers and verify they are passing. I am a little bit concerned that we really need this setup loopback's dependencies. Thoughts here would be helpful.