Commit nest connectors

We implemented the connectors of the pyNN.nest backend which basically forward all connect calls to the NEST simulator. In this course of this, we made the following changes:
- added a function _synapse_parameters() to the projection class which transforms the synapse parameters specified on the pyNN level to the necessary format for NEST
- added a function _connect which calls nest.Connect()
- added a NativeRNG class to the pyNN.nest backend to enable transformation of parameters of random Distribution for the forwarding to the pyNN level.
- adjusted the connect mechanism of recording and stimulating devices to the new nest.Connect function.

Note that these change will only work together with NEST 2.4, where the new connect routines are implemented.