jrubyJar shouldn't require the jar task to execute

[rtyler]

With @mkristian's recent changes being merged, it is now impossible to build a jrubyJar (fat jar) without also producing a jar artifact.

:jar SKIPPED
:jrubyJar

FAILURE: Build failed with an exception.

* What went wrong:
Cannot expand ZIP '/home/tyler/source/lookout/metron/build/libs/metron-3.0+0fd969d.jar' as it does not exist.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

This dependency link should be broken and the jrubyJar task should be able to contruct its tree on its own without requiring that the jar task also execute.

In effect, I should be able to disable the jar task and still execute jrubyJar

[mkristian]

having the JRubyJar from #47 we also need to "extend" project to allow

jrubyJar {
  jruby {
    initScript( 'app.rb' )
  }
}

but on top we want to build "libraries" like

jar {
  jruby {
    defaultGems( 'app.rb' )
  }
}

the first on is similar to what we have with jrubyExec but the second needs to extend or configure the Jar tasks to include those extra resources

[mkristian]

fixed with #47