Fixed in case that plugin directory isn't removed from sys.path
Renamed set_syspath() to activate()
Added is_valid_lib_path() to check for existence of ${VIRTUAL_ENV}/lib/pythonX.X
Added get_vim_special_path()
Fixed activate_venv() to check for the return value of reset_syspath()
Added is_venv_enabled() for checking ${_OLD_VIRTUAL_PATH}

Because deactivate.bat in venv module of Python 3.3 doesn't unset
${VIRTUAL_ENV}
Several fixes