generate unique not random id from string

Problem:
if you commit the styleguide output to a git/svn repo, each change generates random id for _every_ page in the styleguide.

searched for a JS build in function w/o success. In the end the solution from [1] seems elegant. Check [2] for benchmarks.

Resources:
* [1] http://stackoverflow.com/a/7616484
* [2] http://jsperf.com/hashing-strings