Attach HTTP Headers to Ajax calls on RESTAdapter

This is a solution to issue #803.

This simplifies the process of initializing the hash as well as allowing users to set an RESTAdapter.ajaxHeaders object which will be merged to be sent as the settings argument to jQuery.ajax

The user is unable to override any data traditionally set by the old "hash" argument as the hash argument overwrites conflicting values in RESTAdapter.ajaxHeaders by utilizing jQuery.extend