Removes dependency on node.js util module

[deive]

Removes dependency on node.js util module (by copying the only used inherits method to customUtils).

[JamesMGreene]

I'd advise just relying on the NPM module inherits instead. It results in a much smaller browser bundle than including util (as in master), works for non-Node.js environments (e.g. React Native as in #14), and avoids including a custom inheritance method.