Implement vector strength function

As discussed in the mailing list, the pull request implements a vector strength function.

This also includes an improved .gitignore list and a pep8 cleanup of signaltools.py.

You can read more about vector strength here:

http://scitation.aip.org/content/aip/journal/chaos/21/4/10.1063/1.3670512

van Hemmen, JL, Longtin, A, and Vollmayr, AN. Testing resonating vector 
strength: Auditory system, electric fish, and noise. 
Chaos 21, 047508 (2011); 
doi: 10.1063/1.3670512