A couple misc cleanup tasks, mostly around minification

[mattrobenolt]

• Internalized TraceKit._has. It was a private method anyways.
• Created an UNKNOWN_FUNCTION constant since '?' was used several times.
• added _isUndefined for a consistent check
• slightly refactored getSource

[occ]

Travis is complaining :P See: https://travis-ci.org/occ/TraceKit/builds/4659991

[mattrobenolt]

@occ sorry about that. :)

[occ]

Thanks @mattrobenolt :)