added correct NSImage icon support

```shell
Run options: 

# Running tests:

...............................................S.S.....

Finished tests in 1.075285s, 51.1492 tests/s, 136.7079 assertions/s.

  1) Skipped:
test_notify_priority(TestGrowlUDP) [/Volumes/Users/barry/github/steakknife/ruby-growl/test/test_growl_udp.rb:27]:
Errno::ECONNREFUSED: Connection refused - send(2)

  2) Skipped:
test_notify_notify_type(TestGrowlUDP) [/Volumes/Users/barry/github/steakknife/ruby-growl/test/test_growl_udp.rb:39]:
Errno::ECONNREFUSED: Connection refused - send(2)

55 tests, 147 assertions, 0 failures, 0 errors, 2 skips
```