Bug 840819 - Fix Windows Python path not found if it contains spaces

Avoids blocking warning `Failed to find Python installation directory` on Windows when Python is installed in path containing spaces, e.g. `Program Files (x86)`.

See issues e.g. on [Bugzilla bug ID 840819] [bugzilla], [MozillaZine] [mozillazine] and [StackOverflow] [so].

[bugzilla]: https://bugzilla.mozilla.org/show_bug.cgi?id=840819
[mozillazine]: http://forums.mozillazine.org/viewtopic.php?f=7&t=2431553
[so]: http://stackoverflow.com/questions/22872082/error-installing-firefox-addon-sdk-python/25859855#25859855