Only call g.stop() if we are sure g is defined.

This prevents "Device not found" exception being masked as a "undefined g".

(I think the real solution is not to do any real hardware access within
Garmin.__init__ and then call--say--a "setup" method - that would
certainly be the more Pythonic way.)

Signed-off-by: Chris Lamb <lamby@debian.org>
Also link to the github site for compatible pyusb version.

(For various reasons I do not use pip).

Signed-off-by: Chris Lamb <lamby@debian.org>
Merge pull request #66 from lamby/master

Only call g.stop() if we are sure g is defined.