Merge rsroot-env into master

Several changes to the setup script and `rsshell` that have been impatient to make their way into the master branch have finally been tested and made ready for the merge. In particular the logic for detecting the project root directory has been moved entirely to `setup.py` and the new internal subdirectory `/extbin` is added permanently to the user's PATH so that `rsshell` can be invoked at any time from any working directory.

Since a lot happened since the branch forked from `master`, `Readme.md` has also been updated.