Specify Git and PuTTY locations with environment variables

In addition to Git Extensions' existing auto-detection logic it would be nice if the locations of Git and PuTTY could be specified using environment variables.

This is espescially useful in combination with a dependency-injecting package manager such as [Zero Install](http://0install.de), which uses environment variables to tell applications where they can find their dependencies.
That's correct, however operating system calls like `Process.Start()` also use the `PATH` value captured at process start, so I believe this should be the desired behavior.