Managed Fusion URL Rewriter

* Major update to the proxy handler, it is not much faster, and provides an exact duplication of headers from the proxied server. * Added full support for $N and %N support in conditions and rules now. * Added contexts for condition, rule, and ruleset to make transfer of common data easier for implementations of the API. * Added split between async and sync proxy handler, this can now be controlled through the web.config using useAsyncProxy. * Fixed issue with transfer-encoding: chuncked