Include github and issues links in README
Revert change to notify_udp call with icon

Commit @8cc364bd changed notify_gntp to accept an icon but not
notify_udp.  By accident the call to notify_udp was provided an icon,
however.
Test for #11

The failure noted in #11 is due to using a file icon along with UTF-8
characters.  This has been recreated in a test now.
Replace GNTP#load_resource with URI from --icon

There appears to be no need to download the URL as growl will do it for
us if we use the existing URI support in ruby-growl instead of loading
it manually.  This mostly reverts #11, but appears to work exactly the
same.

/c @steakknife