Added fix where creating a new entity wouldn't occur without a name property set

Ran into an issue with Entity creation in the node module where creating a new entity without a `name` or `uuid` property would simply error out without doing anything. I've updated the `Usergrid.Entity.prototype.fetch()` function to  call the callback when there is an error in a similar style to the JS SDK. That way we can check for the `"no_name_specified"` error condition, and create an entity accordingly.