Multi-instance and multi-core

Hi Petri,

Could you please consider merging my changes with your code? I believe these commits are beneficial because:

* setting TCP ports as Maven variables allows to run multiple Solr instances on one machine (by overwriting default ports from command line).

* pom.xml simplification (i.e. preservePath option) makes it easier to implement multi-core solution. Since Solr 4.4 cores can be auto-detected (http://wiki.apache.org/solr/Core%20Discovery%20%284.4%20and%20beyond%29). This approach will allow (after some other changes) to have one "template" multi-core directory.

Kind regards,

Tom