Add opam file

Signed-off-by: John Else <john.else@citrix.com>
travis: Use opam to install dependencies

Signed-off-by: John Else <john.else@citrix.com>
travis: Add the full complement of OCaml/OPAM versions

Signed-off-by: John Else <john.else@citrix.com>
travis: Only test with OPAM 1.2.0

Signed-off-by: John Else <john.else@citrix.com>
Don't call oasis from the Makefile

Signed-off-by: John Else <john.else@citrix.com>
travis: use new OPAM syntax

Signed-off-by: John Else <john.else@citrix.com>
travis: don't install crc, just pin it

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #9 from johnelse/opam-file

Add opam file and make use of it in the travis script