Item Group Support (improvement of #43), including fix for JENKINS-19793 (#44)

This contains changes in #43 and following additionals.
* Backported Items.computeRelativeNamesAfterRenaming from Jenkins 1.530, for work with Jenkins < 1.530.
* Changed the way for avoiding NPE. Does as in getTriggeredProjects (which is reported as JENKINS-19793, #44)
* Added tests for:
** deleting projects with folders.
** fix avoiding NPE. (JENKINS-19793, #44)
** not affected by a bug in computeRelativeNamesAfterRenaming.