Move groovy.icns to lib

Move groovy.icns to lib, to make the OSX install more
compatible with Homebrew.

It's bad practice to include non-executable files in the bin directory. The homebrew maintainers have noted that the issue makes Homebrew's installation of groovy more complicated, as we need to determine which files do and do not need a symlink to /usr/local/bin. Moving the file to lib will avoid that issue.

This patch:

1. Reconfigures gradle to move groovy.icns to the lib folder in the distribution (instead of bin)
2. Changes the startGroovy shell script so that OSX looks in lib instead of bin for groovy.icns