Fixed win32 gui class management

[henryx]

Using as library, it appears this problem:

import ntfy.backends.win32
ntfy.backends.win32.notify("test message", "test")
ntfy.backends.win32.notify("test message", "test2")

Second notify return this error:

  File "C:\Python27\lib\site-packages\ntfy\backends\win32.py", line 28, in __init__
    classAtom = win32gui.RegisterClass(wc)
pywintypes.error: (1410, 'RegisterClass', 'La classe esiste gi\xe0.')

This pull request (tested on Windows 10) fixes the problem

[coveralls]

Coverage increased (+0.03%) to 84.452% when pulling 551c115 on henryx:master into 680891b on dschep:master.

[coveralls]

Coverage increased (+0.03%) to 84.452% when pulling 551c115 on henryx:master into 680891b on dschep:master.

[dschep]

Thanks! LGTM