config.webserver = 'jetty' (by default)

... we're still using an old jetty version which for updating needs us to publish a maven artifact
but it's still newer that winstone, which somehow stayed in 2008

there's some additional cleanup, namely : 
- jruby-rack gem should have stricter requirements
- improved "MAVEN_REPO" detection for cached webserver .jar
- as well as error/stack trace printing

hopefully I get some more time later and continue with : 
- refactoring the packaged jetty if possible (so it can be updated by simply changing a version number)
- review Jar/WarMain there are some stack-traces that should be reviewed (running with *warbler.debug=true*)