Refactoring

Refactor some of the url and request stuff in client.py
Refactor the QueryString class into a function since it was a one-method
class. Removed the "_string" name from the function - you could identify
that it's a string if you needed to know that.
Refactor away some of the mutable default argument handling.

(The tests actually pass now)
Merge branch 'master' of https://github.com/briancurtin/py-KISSmetrics

Conflicts:
	KISSmetrics/client.py
	KISSmetrics/request.py