Fix integer division in gauss_kernel.

-1/2 = -1 and not -0.5 in Python
Merge branch 'feature/stMetrics' into develop

Conflicts:
	spykeutils/rate_estimation.py
Increase performance of gen_homogeneous_poisson

Speed-up factor is about 6.
Improve victor_purpura_dist performance.

Speed-up factor is about 10 to 16.