Shorter way of getting the XHR object

Basically taken from jQuery:
https://github.com/jquery/jquery/blob/1.9-stable/src/ajax/xhr.js#L12-L23
Consoldate function wrapping into a TraceKit.wrap()

This removes a lot of duplicate code.
Refs #3 as well.
A couple misc cleanup tasks, mostly around minification

* 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
Merge pull request #32 from mattrobenolt/better-ajax

Shorter way of getting the XHR object
Oops, debug was used. :)
lol, late night coding
Merge pull request #33 from mattrobenolt/tracekit-wrap

Consolidate function wrapping into a TraceKit.wrap()
Merge remote-tracking branch 'mattrobenolt/cleanup'

Conflicts:
	tracekit.js
Merge branch 'master' of https://github.com/occ/TraceKit