Minor bugfix for Python spike queue with subgroups
Fix an "off-by-one" error in handling subgroups in the C++ spikequeue that could lead to segmentation faults.
Add a "conditional write" attribute to variables annotated with "unless refractory"
wrote some code to use .take instead of [] in numpy code, but left it
commented for the moment, cf: http://wesmckinney.com/blog/?p=215
Added CUBA example from Brian 1
Some progress towards conditional_write mechanism in numpy, but still
plenty to do

Current status:
- dev/ideas/conditional_write.py works
- 2 tests fail, but this isn't surprising because refractory behaviour
has changed
- 9 tests have errors, with _presynaptic_idx and _postsynaptic_idx