Chain for 3rd-party deps resolution (LAS-40)

Changed 3rd-party library dependency resolution from one single proprietary repository to a chain, working as the following: first look, if dependency can be resolved via Maven Central. This will be true in most cases. In particular for LAS2peer-specific libs, not yet stored in Maven Central, the chain then includes the LAS2peer-specific Apache Archiva installation. (LAS-40)