Added JSHint to look for potential bugs

[JSHint](http://jshint.com/) is a nifty tool for identifying potential sources of bugs. For example, JSHint run on backbone shows:

    $ jshint .
    backbone.js: line 85, col 41, Expected an assignment or function call and instead saw an expression.
    backbone.js: line 119, col 20, Expected a conditional expression and instead saw an assignment.
    backbone.js: line 249, col 29, Expected an assignment or function call and instead saw an expression.
    backbone.js: line 254, col 46, Expected an assignment or function call and instead saw an expression.
    backbone.js: line 255, col 18, Expected a conditional expression and instead saw an assignment.
    backbone.js: line 304, col 29, Use '!==' to compare with 'null'.
    backbone.js: line 312, col 15, Use '===' to compare with 'null'.
    ...

I left the code alone out of respect for the backbone team; feel free to resolve these by modifying the code according to the JSHint recommendations, or modify `.jshintrc` to hide such warnings.