Created space-in-brackets rule

Created the `space-in-brackets` rule, which enforces spacing
preferences within brackets:

    var obj = { foo: 'bar' };
    var arr = [ 'foo', 'bar' ];
    foo[ 'bar' ];

    var obj = {foo: 'bar'};
    var arr = ['foo', 'bar'];
    foo['bar'];

The rule takes one option, a string, which may be either "always"
or "never".

Fixes #631
Merge pull request #640 from iancmyers/rule-space-in-brackets

Rule: space-in-brackets