Allow pre/post references in synaptic expressions

This branch introduces the possibility to use `_pre` and `_post` references everywhere in synaptic expressions, i.e. for creating synapses, setting synapses and indexing synapses. I did implement in the most simple way by introducing new templates specifically for the `Synapses` class. These are very reduntant and should simplified as mentioned in #94.

Closes #96.