Proxy Support, fixes #102

I took a shot at implementing proxy support as suggested by @jrudolph.

 - I patched ```HttpHostConnectionSlot``` to take an optional proxy parameter.
 - The new ProxySettings can be read from system props or configured directly.
 - The logic for if/which proxy settings are used is implemented in the ```HttpHostConnector```.

Again suggestions on how to further test this are appreciated...

Overalll this still feels a bit hacky. Maybe some kind of ```ProxiedHttpHostConnection``` as a simple wrapper around ```HttpHostConnection``` that connects to the proxy connection would be more elegant.

Only plain HTTP is supported ATM.