Modified pynipap to not store XML-RPC connections

To avoid the issue with XML-RPC-connections never being closed, XML-RPC
connections are no longer stored in each instance of the NIPAP objects.
The impact of this will probably be small as only the save() and
remove()-calls actually used the stored connections.

Fixes #384
Merge branch 'master' of github.com:SpriteLink/NIPAP