Add opam file

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update travis to use opam 1.2 and avoid hard-coding the depends

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
travis: build with 4.01 and 4.02 OCaml versions

This is mainly because Mirage needs 4.01 to work.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #9 from djs55/fix-travis

Update travis to use opam 1.2