(#11714) Use %~dp0 to resolve bat file's install directory

This commit uses the %~dp0 batch script modifier to resolve the
drive and path of the directory containing the envpuppet.bat
file. This eliminates the need for hard coded paths within the script
itselfIt also uses %VAR:\=/% to substitute each backslash for a
forward slash in the RUBYLIB environment

Also added a section about running the spec tests on Windows.

Reviewed-By: Jeff McCune <jeff@puppetlabs.com>