STDP reintegration

Integrated latest implementation of STDP from SVN, at this point, features are:
* All-to-all and nearest neighbour pair-based STDP rules
* Pfister and Gerstner's triplet rule
* Additive weight dependence
* Two STDP examples - reproduce reasonable well known protocols from the literature on SpiNNaker
* Take any maximum weight specified by the STDP weight dependence into account when calculating weight scaling

When you get a minute, @rowleya  come and talk to me about this!