monitors.py: Propagate spikes only if spikes exist

[mstimberg]

Some of the monitors (I noticed it for SpikeCounter) process the spikes in every time step with statements like:

self.count[spikes] += 1

This takes considerable time even if the spikes array is empty. I'd therefore suggest to do it as in the Connection classes and put an "if len(spikes):" around such code.

Attached a patch (I also added some tests to increase the test coverage a bit ;) ) -- good enough to commit or do you see any problems?

[mstimberg]

Patch to process the spike array only if it is not empty

[mstimberg]

Romain said "it looks reasonable" -- commited as [2755]