Heavy client refactoring, and adding support for async requests.
Added a lot of unittests.
Code coverage ignore
First time using 'goto'! yay :)

A valid usecase I think. Normally I would just call continue in a
situation like this (which is also a form of goto, if you think about
it..)

But giving it a label and using goto is actually clearer here. No need
for counting while, fors and whatever to figure out where we're
continuing towards.

This commit also fixed a number of async bugs. It's tested now
Added an example for async calls.
Updated changelog. Removed some whitespace.
Merge branch 'master' of github.com:evert/sabre-http