proxy: Refactor so that other upstream backends can be implemented

adding crypto/tls sessioncache

Refactor proxy middleware so that 1.) From() is exposed 2.) Other upstreams can be implemented/plugged in

Proxy Middleware: Add support for multiple backends, load balancing & healthchecks

config: format string fixes from vet

fastcgi: allow more request types.

Upgrade proxy middleware. Add support for: multiple backends, load balancing, health checks, and pluggable backends

git: post pull command. retries after pull failure.

fastcgi: user defined environment variables

Specify Go Version in README

Implementation of Git middleware

Defaults path to site root.

If we do not use a pointer here, the startup function that opens the log file stores the log file in a copy of the handler, not the same instance of the handler, causing panics during requests, which is bad, especially when the response is gzipped: the next recover() is beyond the gzip handler, so the browser downloads a gz file instead.