Added: ssh proxy command support and additional parser arguments

In case your ffmap-backend server is not part of the mesh you can use ssh to tunnel to your batman mesh:

```
bat2nodes.py -m bat0 -s root@10.0.0.1 -d ../ffmap-d3
```

Additional: support for dhcp.leases file (for aliases) and possiblity to skip rrd database.

To generate ffmap data (without rrd) I use the following snippet

```
scp 104.X.X.X:/tmp/dhcp.leases leases
python2 bat2nodes.py -m bat0 -s root@104.X.X.X -d ../ffmap-d3 -n -l leases
```