provide OSHttpRequest and OSHttpResponse to our REST handler.
also, this adds proper RestPlugin.IsGod() checking against the X-OpenSim-Godkey
HTTP request header.
last, i added XML doc comments to RestPlugin.cs
takes a look at the HTTP request header and retrieves x-opensim-godkey (if it exists)
and compares that against the godkey from OpenSim.ini.
also, this makes AddStreamHandler() virtual so that it can be overridden.