Fixes problem in WeightPatternMonitor

Was only recording half of the relevant connections. Now fixed.
Removes unneeded include statements from examples
Adds stopping time to VoltageMonitor
Adds new recording mode to WeightMonitor

This recording mode called GROUPS resembles the functionality of
WeightPatternMonitor and allows to now recorde the statistics mean/stdev
of all or a subset of synapses between one group of neurons (pattern) to
another (pattern).
Adds new functionality to StimulusGroup

StimulusGroup can now also generate patterns with a random stimulus
strength which is normal distributed for now -- although we might want
to change this later.
Merge branch 'master' of https://github.com/fzenke/auryn
Removes doc/README