add module for ipv4 uplink via gre tunnel and bgp

as freifunk rheinland offers it to communities.

this was tested and works as far as I can see.
please merge #51 first and have a look at #37 afterwards
Deactivating rp_filter by default for all devices is no good idea. 
Better deactivate it per device like in files/etc/openvpn/anonvpn-up.sh