Configured JSHint to look for trouble spots

[JSHint](http://jshint.com/) is a good way to scan JavaScript code for areas that might lead to bugs and unexpected behavior.

I ran `jslint .` on the latest underscore and saw 1094 lines that could use a bit of improvement. Most of these are trivial to fix, but I thought it would be more respectful to let you guys decide how to resolve them--you can even turn on and off certain JSHint rules in `.jshintrc` to really fit what you care about in your codebase.

Example:

    $ jshint .
    test/arrays.js: line 6, col 76, Mixed double and single quotes.
    test/arrays.js: line 7, col 68, Mixed double and single quotes.
    test/arrays.js: line 8, col 40, Mixed double and single quotes.
    test/arrays.js: line 8, col 75, Mixed double and single quotes.
    test/arrays.js: line 9, col 40, Mixed double and single quotes.
    test/arrays.js: line 9, col 49, Mixed double and single quotes.
    test/arrays.js: line 9, col 79, Mixed double and single quotes.
    test/arrays.js: line 10, col 40, Mixed double and single quotes.
    test/arrays.js: line 10, col 52, Mixed double and single quotes.
    test/arrays.js: line 10, col 82, Mixed double and single quotes.
    ...
    1094 errors