fix plugin extension point, enhanced jelly script, added test connection button

.classpath
  Added eclipse classpath to web resources

pom.xml
  Update jenkins dependency to ver 1.502

src/main/java/hudson/plugins/gearman/Constants.java
  Added default setting for launchWorker configuration.

src/main/java/hudson/plugins/gearman/GearmanPluginConfig.java
  This plugin was extending the wrong jenkins extension point.
  It's now extending GlobalConfiguration instead of Builder which I
  believe is more correct. I had to rename GearmanPlugin to GearmanPluginConfig
  because for some reason extending GlobalConfiguration from a GearmanPlugin
  class doesn't work. Also had to rename the associated jelly script from
  global.jelly to config.jelly

  Moved and renamed help files to let jenkins framework load help files named help-$field.html

src/main/java/hudson/plugins/gearman/ManagementWorkerThread.java
  changed debugging from System.out to logger.info

src/main/resources/hudson/plugins/gearman/GearmanPluginConfig/config.jelly
  Added basic verification for host config (make it required).
  Changed port config type from textbox to number and added basic verification (make it a required number)
  Added a 'Test Connection' button to test the Gearman server connection.
  Remove references to help files to let the jenkins framework load help files named help-$field.html

src/main/resources/index.jelly
  Update description

Change-Id: If0a521c8611d6a93f0434000452dc8c284f5fd74