Add serverconnection scripthook

Add server connect script hook as requested here: https://groups.google.com/forum/#!topic/mitmproxy/6FDkWfrh3HE

*Aldo*: We have ClientConnect and ClientDisconnect vs ServerConnection classes in mitmproxy. To provide an unified API, I opted for a serverconnect scripthook. Are there any planned internal refactorings? If there are no obligations from your side, leave a comment and I'll add docs and merge.

Cheers,
Max

PS: Branching with multiple remotes sucks, PR https://github.com/mitmproxy/mitmproxy/pull/168 is included here. I apologize! As there are no significant changes in it, I let it just as is...