Disabled model factory injections by default.

Given that it is currently very common to use
models as globals, and that
`App.Post !== lookupFactory('model:post')` when
factory injections are enabled, it made sense to disable
model factory injections by default.

Added the flag Ember.ENV.MODEL_FACTORY_INJECTIONS
to enable model injections.

Model factory injections will become the default
when accessing global models is no longer a common pattern.