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.
Refactor some of the url and request duplication in client.py
remove url attribute from Client
use dict.update rather than an unnecessary loop
Merge pull request #12 from kissmetrics/brian_curtin

refactors thanks to @briancurtin and @njm