added support for TLS when proxying via wai

Hi Michael - 

I quickly hacked together the ability to reverse proxy to a TLS endpoint (simply enabling the built in support by HTTP.Conduit). I'm not convinced that I've made the changes in the best manner possible, as i've only concentrated on the waiProxyTo functions (not rawProxyTo). Would be cool if you could take a look and see if this approach, or something better could be incorporated into the main project.

Thank you for your great work, I'm a newbie haskeller, but i've learned a great deal going through some of your code (especially ResourceT!) :)

Ciao,
Tom