Pass a ref into the url property

[davideast]

This PR address the ability to pass a Firebase reference to the url property in either a model or a collection.

[coveralls]

Coverage remained the same when pulling ca2169b on url-ref into 40ad2c7 on 0.5-release.

[coveralls]

Coverage decreased (-0.79%) when pulling 4dfa7e4 on url-ref into 40ad2c7 on 0.5-release.

[coveralls]

Coverage remained the same when pulling 60530c1 on url-ref into 40ad2c7 on 0.5-release.