Bind mount implementation and tests

Despite the well-intentioned reluctance to this feature expressed in #111, being able to mount host directories on containers is critical for many use cases.  I've completed a naive implementation (with tests). I hope we can get something like this in master soon!

Usage is as follows:

    docker run -b /host base ls /host                     # mounts /host through to the container
    docker run -b /host:/container base ls /container     # mounts /host to /container
    docker run -b /host:/container:ro base ls /container  # mounts /host to /container in read-only mode

The bind mounts default to read-write but also support read-only mode.  I worked this out by modifying `lxc.mount.entry` manually and working backwards.  Given my unfamiliarity with the code-base, this should get close review before being merged.