Rule: no-dupe-keys

The `no-dupe-keys` warns on the use of duplicate keys when declaring object literals:

var foo = {
    bar: "baz",
    bar: "qux"
};

Fixes #40 Fixes #44 Fixes #154 Fixes #155
Merge pull request #195 from iancmyers/rule-no-dupe-keys

Rule: no-dupe-keys (Fixes #40 Fixes #44 Fixes #154 Fixes #155)