Added option to restrict remote repository access to specific path(s)

With this option remote repository access can be restricted to a
specific path for a specific ssh key using the following line
in ~/.ssh/authorized_keys::

command="attic serve --restrict-to-path /data/clientA" ssh-rsa clientA's key
command="attic serve --restrict-to-path /data/clientB" ssh-rsa clientB's key

Closes #51.