implement persistent storage specification

A new option is added for ``docker run``: -volume.  It allows to mount a host
directory within a container.

Usage:

    -volume /container/path=/host/path

Note: both paths must be absolute
make it possible to specify ro/rw for mounted volumes

The specification of the volume to mount is changed from

    -volume /container/path=/host/path

to

    -volume mode:/container/path=/host/path

where mode is either 'ro' or 'rw'

Fixes #111