Added a first version of Synapses with the possibility of dynamically updated delays.
The way this is done is mostly by having delays act as a simple attribute that is used to update the spikequeue at each timesteps.

It only works now when the dynamical delays are: a) presynaptic b) the only presyn spikequeue (there can be multiple)
I'll try to improve this.

See you in a minute when this breaks all the tests and I have to resubmit a patch.