Feature - Add possibility to export shared methods to client models

This PR adds the possibility for server models exported to Angular ressources, to define specific methods which will be exported as well to be exposed in the client.

The models need to specify the shared methods in an array property called `clientMethods`

Example :

    MyModel = app.model('Brand', schema);

    MyModel.prototype.getUrl = function() {
        return '/#/mymodels/' + this.id;
    };

    MyModel.prototype.clientMethods = ['getUrl'];

After using loopback-angular with the above model, the `getUrl` method will be made available on the lcient side for every instance of `MyModel` objects

Note: I added tests to the specs for this behavior.