Allow pre/post references in synaptic expressions

[mstimberg]

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.

[coveralls]

Coverage remained the same when pulling b16ff91 on fix_pre_post_references into 2105e39 on master.

[mstimberg]

The example I just added shows how this mechanism can be used for spatial connectivity patterns. Quite neat I think!

I've nothing to add for now, from my side it's ready to merge.

[coveralls]

Coverage remained the same when pulling f6b19db on fix_pre_post_references into 2105e39 on master.

[coveralls]

Coverage remained the same when pulling 7f87c03 on fix_pre_post_references into 2105e39 on master.

[coveralls]

Coverage remained the same when pulling 5966ed1 on fix_pre_post_references into 1b780ee on master.