Created Class nest_build_nineml_synapsetype to read synapses in nimeml format

[pedroernesto]

• Added Class SpikePairRule_ in pyNN/nest/standardmodels/synapses.py, to include more parameters, like Song & Abbott STDP synapse model

• Created Class _nest_build_nineml_synapsetype inside file pyNN/nest/nineml.py, to read synapses in nimeml format, and compile the code with pype9. Compiled files are stored inside dir examples/NEST_nineml_models

Class _nest_build_nineml_synapsetype returns a nineml_synapse_type object, that is called from examples/simple_STDP_nineml.py

Note: Some Jinja template files are involved in the code generation process above. They must be filled in with NineML model representation data, by means of pype9/blob/plasticity/pype9/simulate/nest/code_gen/base.py