Documentation updates for Windows

Everything seems to basically work on Windows. Update the README.md and
config.example.coffee with some hints for Windows users.
Create git-util.coffee

This contains spawnGit and gitOutput from git-blob.coffee which are
usable for any Git command
Reuse running git rev-parse instances

Instead of spawning a git rev-parse instance for every request look to
see if one is running and reuse that instead. Tests with loading a
simple page showed >50% reduction in spawned rev-parse instances.

Besides performance this helps with Mac OS X's draconian process ulimit
Prevent crashes when HTTP clients hang up

At the same time take a less drastic approach to stopping git cat-file.
It should notice that its stdout went away