Enable calling 'rsshell' without the '.py' extension on Windows

If setup.py is run on Windows and it detects that '.py' is not
included in the PATHEXT environment variable, it will insert a small
additional batch script in ROOT\extbin with the name 'rsshell.cmd'.
This script simply calls extbin\rsshell.py. Since '.cmd' is always in
PATHEXT, rsshell can now be called without needing to type the
extension.
Minor nitpicks in setup.py
Add /wait option to rsshell.cmd
Merge pull request #22 from jgonggrijp/rsshell-ext

Enables Windows users to run `rsshell` without needing to type the .py extension.