Adding a hash method to underscore.string.js

# A hash method for underscore.string.js
I needed a string-to-hash method for my current project to create keys for strings and to add a simple password check without transferring the password as plain text to the server. I think this hash method for strings fits perfectly with underscore.string - actually, I was a bit surprised that it did not exist already, neither here nor in underscore.js.

# About the added method:
```_s.hash(str)``` returns a number unique for the given string. The method is the same as implemented in Java and thus a kind of standard. It can be used for creating IDs; avoiding plain text when checking or transferring passwords or for caching. The implementation has been chosen according to http://jsperf.com/hashing-strings/22 (the one with the highest performance on the majority of browsers).