Update OASIS and opam packaging to add a Lwt_vmnet and ocamldoc
Update the bindings to not callback directly into the OCaml runtime

Instead, we signal from the GCD callbacks to a condition variable
that unblocks an OCaml thread.  This is because there is no 1:1
mapping between GCD queues and pthreads, and so registering each
of them with the OCaml GC is problematic.  With this solution, the
only communication back to the OCaml GC is via a single condition
variable.
Add Lwt_vmnet module that wraps Vmnet using Lwt asychronous I/O

This uses Lwt_preemptive under the hood to detect when the vmnet
interface is ready for reading, since there's no pure select-based
alternative.