Re-add OASIS

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add explicit license file (in addition to existing code comments)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
travis: xen-block-driver -> mirage-block-xen, add io-page

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #5 from djs55/add-oasis

Switch build back to OASIS