Build notes file path

I found that it was easier to manage the build notes as a file in my repository rather than having to go into Jenkins to write them into the Job's configuration field. This way they are version controlled and can be created in the IDE or other editor along with the code. I realise it may be possible to do this using environment variable hackery, but I couldn't get newlines to work with the EnvInject plugin, so this ended up being quicker and I think nicer.

It will look for a BUILD_NOTES file in the workspace root by default, or if a file is given it will look for that first on the file system, then inside the workspace.