Remove XEngine.m_uniqueScripts

This provides no obvious benefit as far as I can tell but has a definite cost in keep script strings in memory for the entire simulator session.
Adding command to reset user's email address
Add command to reset region's user cache
Fix recent regression where a race condition meant SP.MakeRootAgent() would sometimes look to start attachment scripts before ETM.HandleIncomingSceneObject() had added them.

Probably a regression since ghosts branch merge on Nov 26 2014
In XBakes use proper disposal semantics to always close the stream on Store() no matter what
Use using() semantics in XBakesModule to ensure disposable objects are alway disposed no matter what
minor: Correct propery doc RestClient._resource spelling mistake
In XBakesModule.Get() use using() to always dispose of RestClient which disposes the stream rather than disposing the stream directly
Make sure we always dispose of disposables inside RestClient.Request()