Return boolean on update and create

Return boolean on update and create to avoid backbone sync resetting each attribute. reference: https://github.com/jashkenas/backbone/blob/master/backbone.js#L509

Solution Caveats:
 - String coercion will not be performed on save() as it is now so in memory models could vary from localStorage counterparts.

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I came across this using the current implementation of Backbone.localStorage and Backbone-relational. I was listening to events on child relations in parent models, which of course were unattached during save() operations since the child relation attribute was being overwritten with the 'serverAttrs'.

I am submitting this pull request as a proof of issue/resolution, but the actual architectural improvement may be better solved in Backbone itself.