IPND Draft 02

Implementation of IPND Draft 02 for service discovery. To enable, set `discovery_version = 3` in `ibrdtnd.conf`.

All CLAs currently supported by IBR-DTN can be announced, but only TCP and UDP CLAs are specified in the current draft. The other services were implemented as private service types (according to [section 2.6.2](http://tools.ietf.org/html/draft-irtf-dtnrg-ipnd-02#section-2.6.2) of the draft) and are most probably not portable across DTN implementations.

The current implementation does not yet handle Neighborhood Bloom Filters, also compatibility to DTN2 was not yet tested. Both should change in the next few days, but until then, the code is already up for comments.