Rule: no-empty-class

The `no-empty-class` rule disallows the use of empty character classes in regular expression literals:

var foo = /^abc[]/;

It does not disallow empty character classes in the `RegExp` constructor:

var foo = new RegExp("^abc[]");

This rule matches the JSLint rule: http://jslinterrors.com/empty-class/
Merge pull request #264 from iancmyers/rule-no-empty-class

Rule: no-empty-class