Better diagnostics and visibility for trace support

 * export echoTraceSupported key indicating whether
   echo:run and echo:backgroundRun will work or throw
 * export echoAkkaVersionReport and echoPlayVersionReport
   which can be used to display why tracing is not enabled
 * improve logging when run fails and when scanning the
   classpath, using the version reports
Merge pull request #9 from typesafehub/wip/havocp-version-support

Better diagnostics and visibility for trace support