Plumb through Vchan peer UUID registration so that nodes can have names

This permits each configuration Conduit to specify a name for its
local node, which is subsequently used in Vchan lookups
Travis: Add a temporary shim in to build via avsm/ocaml-conduit
Travis: use opam 1.2.0
travis: fix pin syntax for opam 1.2
Add a vchan_default which selects the right Xen/Unix implementation
Rename the Resolver to Resolver_direct and refresh the API
Add a Unix system resolver variant
Travis: remove the temporary avsm pin
Merge pull request #315 from avsm/vchan-peer

Plumb through Vchan UUID registration