opam: Add build-test section

Signed-off-by: John Else <john.else@citrix.com>
opam: Pull out separate install section

Signed-off-by: John Else <john.else@citrix.com>
opam: Test the docs build

Signed-off-by: John Else <john.else@citrix.com>
travis: Use xapi-project fork of the standard skeleton script

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #11 from johnelse/use-travisci-skeleton

Tidy opam packaging and use travisci skeleton