Add Async support

[georgyo]

This adds Async support, it has been tested by using these modules in the Churrua DHCP server, and works well.

Since Async depends on Core and ppx_jane already, I used that in the rawcap_async test binary. It could just as easily use Async.Deferred.run_in_async_wait instead of the Command module to start the process. If there is desire to keep rawcap_async as minimal as possible.

[haesbaert]

That's great, I'll have a proper review in this and all the other PRs tomorrow, I'll test this during the weekend in my network home to see if nothing breaks

thanks again for your work