Add namespace for resource URL

[tshi0912]

Currently, there is no option to specify the namespace for resource URL. People can reopen ResourceAdapter to add namespace:
Ember.ResourceAdapter.reopen({
ns: 'json/v1'
});
However Ember-REST still need to be changed to accept this namespace:

_resourceRequest: function(params) {
params.url = this.ns + this._resourceUrl();
...
...
}