virtual machine mounts

This patch adds a MountObserver class which explicitly uses polling only
and ignores Inodes.

Although this is no all-purpose solution, it makes watchdog work reliably
on drives mounted from virtual windows machines, where the simulated
inodes are changing all the time.

Also I want to motivate to create a new version on PyPI.
There were lots of patches since over a year but PyPI still has the old 0.6.0 version.

For that reason, I created a temporary version _pydica-watchdog_.
I would be happy to remove that, ASAP.