Bug 946813 - Part 2: Expose invisibleToDebugger via the loader.

https://bugzilla.mozilla.org/show_bug.cgi?id=946813

In part 1 of bug 946813, I exposed a new sandbox option "invisibleToDebugger" which marks the sandbox's compartment as invisible for debugging purposes, which eventually will help me resolve issues with having two debugging servers active at the same time. Since we load the server via the add-on SDK loader, I need to expose the ability to set this flag on all the sandboxes one loader creates.