Operate on a shared object space from two threads concurrently

[tobbebex]

Cropped off from #18.

Add a fork method that operates in ContextT m monad.

Challenges / Requirements:

• The main thread must outlive the other threads to provide window handlers a mean to RPC various main thread only commands.
• All GPipe state need to be synchronized in a threadsafe way (We need to allow users to send GPipe objects in MVars between the threads).
• All OpenGL state need to be synchronized between the threads using glFinish and/or fence objects.

[plredmond]

Might close this depending on discussion in #77.