Feature: enable parse of model-config.js for client models extension

This PR enables `model-config.json` definitions for the angular client *ie* enables models code sharing within current angular-sdk-client

I implemented this because full-blown browserification (like in loopback-example-full-stack) is a too heavy payload for me, and because `loopback-sdk-angular` works well for me.

**3 main commits with comments on each of them for the details**

### List of main features added by each commit
  1. 0cdf152 Enables client-side methods from sources defined in `model-config.json`
  2. f9587d4 Enables non persisted models to be shared with the client as well (not angular resources, just classic objects)
  3. 3ed6668 Enables declaring Dependencies used in each resources custom methods

### Highlights
 - No compatibility break
 - Specs updated 
 - Tries to respect loopback 2.x workspace structure / model config
 -  Not really isomorphic, but for simple needs it's easier and lightweight
 - Just drop client code in client/models, common code in common/models, and *voila*

### Notes
  - I added a fix for the warnings about deprecated usage of `res.send` express method, can remove if out of purpose
  - I have updatet `grunt-loopback-sdk-angular` as well, incoming PR

Related: the issue #24 and a previous attempt on the implementation in #25.