Jamie plasticity

So it's all there in time for the code-freeze, this pull-request is basically just support stuff for the learning rule I'm working on with Simon https://github.com/SpiNNakerManchester/sPyNNakerRecurrentSTDPPlugin and BCPNN, this includes:
* Fixes to the PlasticWeightControlSynapseRowIo class (used for writing plastic synaptic rows where there's an extra half-word of control data per-synapse) so it works!
* Fixing support for taking signed, plastic synaptic weights into account when calculating ring-buffer scaling
* Removed tau_plus and tau_minus from AbstractTimeDependency as they aren't part of many time dependancies
* Added some more build targets
* Added command-line options to build executables with very simple synaptic event processing benchmarking built in
* Support for an intrinsic plasticity term in neural models (could also be used for noisy current inputs etc)
* Fixed multiplicative weight-dependence