Use Maven Tycho to build pmd-eclipse-plugin

This pull request tries to streamline the release process of the pmd-eclipse-plugin.
It uses tycho to perform the build, so that you can simply run `mvn clean install` and you have a installable plugin under `.../target/repository`.

I've also written a short howto for the release process and publishing the new plugin version to our official eclipse update site (http://pmd.sf.net/eclipse/).

For the new documentation, see here (also included in the pull request):
https://github.com/adangel/pmd/tree/eclipse-tycho/pmd-eclipse-plugin