Simple balancing, simpler Spring Configuration, Header-Modification

Hi, 

I implemented some features that we need for a project:

1) Support for modifying request headers within the proxy (class HeaderModifications)
2) A simpler Spring configuration (classes SpringHTTPProxyServlet and SpringProxyFactory)
3) A simple load-balancer (class BalancingProxy)

You can merge it if you want. Sorry though, for messing with the whitespaces in the POM.

Regards,
Nils