improve owncloud --version

Currently it prints 1.7.0 although we are in beta state before 1.7.0 -- should it print 1.7.0~beta3 instead?
Suggested by this PR.

Note that I also suggest prefix MIRALL_VERSION_SUFFIX  in VERSION.cmake with a debian style '~' character. Maybe OwnCloudCPack.cmake should also do that?
Also made MIRALL_VERSION_STRING show the GIT_SHA1 when no numeric BUILD number is there.
This needs a move in CMakeList.txt -- hopefully without other side effects.

Improving on https://github.com/owncloud/mirall/issues/2211 (somewhat optional, but hey)