Teal 13 17 acu134044 support mingw

@szmyd changed RightGit::Shell::Default from a module to a singleton class because the semantics used didn't work going from ruby 1.8 to ruby 1.9
added support for Shell::Default#execute :set_env_vars and :clear_env_vars options to use non-string keys and values because some existing packaging code expected this behavior
more better specs