Bump version to 1.9.13 and create a changelog

1.9.13 is a fairly arbitrary number, but it's the number that Dave Scott is
using in his Project Fusion branch (xapi+libvirt for ceph support). We should
be converging on 2.0.0 soon, but we don't know what we'll put in that version.
We've added a changelog as well, which we will commit to start populating
regularly.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>