Get all additional deps from "Structr Central"

These changes are mostly about the introduction of a "Structr Central" Repository to concentrate all additional dependencies to one repository. These are all placed on my Nexus instance at http(s)://projects.aschemann.net/nexus. This could also be moved to another repository server controlled by the Structr team. 

This has finally enabled me to build everything on a Cloudbees Jenkins instance for continuous integration (this was broken before due to dependencies which could not be found in the repositories/caches).

It is, however, always a good idea to set up a central repository (corporate repository) under full control. Otherwise it is possible that builds break if the used dependencies vanish from "the Internet" some time in the future.

The merge also 
- contains some small enhancements to enclosed shell programs (not completely tested up to now)
- cleans up some of the POM structures (centralizes Repository, SCM, ...)