Additional changes for #43

This is an additional changes for #43.
Please review and consider merging this.

Changes are:
* Backported `Items.computeRelativeNamesAfterRenaming` from Jenkins 1.530 (seems not released yet...).
    * For users using Jenkins < 1.530 (including me!).
* Changed the way for avoiding NPE. Does as in `getTriggeredProjects`.
* Added tests for:
    * deleting projects with folders.
    * fix avoiding NPE. (It fails without your fix)
    * not affected by a bug in `computeRelativeNamesAfterRenaming`.