Altered windows node stop functionality

This now takes advantage of the stop-server.bat script which allows for
graceful shutdowns on windows. The old logic is preserved for previous
versions
Moved windows node stop to its own function

In testing there was an intermittent issue where stop-server.bat did not
properly kill cassandra. If that occurs, the old windows kill method is
reverted to.
Delete pidfile when node is stopped on windows

When a node is started, it checks for the pidfile to be written by
cassandra.ps1. If the pidfile from an old C* process remains, it is
possible for the old pid to be read which prevents the new node from
stopping, as it no longer knows its correct pid
Merge pull request #123 from ptnapoleon/master

Implemented graceful shutdown of C* on Windows