Support gem installation of shoes on Unix systems

Addresses #231 as I understood where things landed. The story (this is also present in the ext/Rakefile with pointers to it from other odd bits of the code):

Running JRuby on OS X requires options passed at the start. To pass those options and avoid double JVM startup, we want 'shoes' to be a bash script on Unix systems, but Rubygems only supports installing Ruby to the bin.

We work around this by installing our script as shoes-stub, then acting like a "native extension" via this Rakefile, we manually generate the right script (a copy of shoes-stub) in the Gem.bindir.

We also provide a Rubygems plugin to clean up this manual file on uninstall. Post-install hooks didn't seem to work, though, hence the extension trick.


FWIW, not sure but we probably need to pull something similar if we're on a Windows system, but I don't have one available to take a whack at that right now.