Compatibility with Backbone 0.9.10 (fixes #9)

There were two additional problems besides the one in issue #9 :
* `this._byCid` does not exist anymore in 0.9.10
* `!model.id` breaks with a numeric id set to `0`

Using the `get` method here is much cleaner as it is guaranteed to abstract between the different versions of Backbone. Besides, not depending on private interfaces is always good :wink: 

As @Crisfole said, `this.models` was not initialized properly.

The test suite now passes with both Backbone 0.9.10 and 0.5.3.