Move sbt sample to its own directory.

We are making room for Maven sample that will be added to this repo.
Add Maven sample.

Add a pom.xml that:

  * defines `scalaVersion` property
  * shows how to add dependency on scala-xml
  * contains work-around for SI-8358 (conflated classpaths)
Update README to reflect that we have two samples now.
Simplify conditional module dependency in sbt sample.

Per @adriaanm's suggestion, I boiled down the logic to its essence.
It still supports Scala 2.11 and newer versions.