Part of #1139

The goal is to help getting started with customizing / enhancing PMD.

• PMD Examples - https://github.com/pmd/pmd-examples
• https://github.com/pmd/pmd-examples/tree/apex-only-dist
• More step by step guides ("walkthrough")
• TOOD: which ones are missing?

I've now created the start of a pmd-examples repository, which hopefully makes it easier to get started when one wants to customize PMD with own rules.
These guides should become part of the official documentation under pmd.github.io.

What's missing:

• more explanations, especially for the maven example: Most stuff works out of convention, so we should make clear, what the conventions are (especially test cases).
• a gradle example
• general plain PMD usage examples for maven projects, gradle projects and ant projects
• easier way to update to the latest PMD version, maybe autodetect?

Note: I've also managed to get a travis build running for this repo. I envision this examples repo as a early notification if somethings break. Although - to be really early, we would need to run against a snapshot version of PMD...