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
Internally thats what its doing, externally its a result of an operation (which Growl calls the state). GrowlRequestState might be a better name for the class. The whole process feels a bit awkward, but thats more to do with what the Growl library does/how it handles things, not the code you've written.