Gnttab and Gntshr may allocate pages or not

One style of usage involves us passing in buffers and grants,
while the other involes us being returned buffers. We support
both kinds.

Signed-off-by: David Scott <dave.scott@citrix.com>
bump to 2.0.0
Merge pull request #11 from avsm/minios-gntmap

Gnttab and Gntshr may allocate pages or not
I seem to be in copyright header for this, so add myself to Authors list
Increase test matrix to OPAM 1.2.0 as well
Add a local `opam` file for supporting the OPAM 1.2.0 workflow