Fixed several issues in GrowlService:

1) Calls to the growl SDK were not made in a thread safe manner

2) Loading the icon from a URL did not work in mono, and it made
   the GrowlService calls take exceptionally long

3) The result of the growl SDK calls was not checked

4) A bug in the growl for windows SDK caused the mono version
   on unix platforms to not work. A fixed version of the SDK is
   now included in the build (do not use the NuGet version, it's
   broken on unix). The bug was reported to growl for windows here:
   https://code.google.com/p/growl-for-windows/issues/detail?id=48
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