Changed __init__() if NetworkConnection.

It now just does nothing instead of raising an Exception, as the method should
be called by its base classes.
Moved Location classes into separate file.

They are now in path.py instead of filesystem.py. FullLocation no longer
contains a mountpoint member, since this is now incorporated into
Device. The user, host and path members were outsourced into a new class
called Location, which is also the base class of FullLocation, that
represents a full path on a host without device information.
Adding mountpoint member to Device.

Now it's no longer necressary to have a (Device, Mountpoint) tuple, but every
device has information about a mountpoint and holds methods to mount the device
to that mountpoint.
Renamed class and method.

CronTab was kind of misleading, as it only holds one single cron string.
CronJob is apter.
Adapted autobackup to new interfaces.