Fix #13 - Release a version compatible with Maven 3.1.0

Hide API changes between Maven 3.0.x and 3.1.x using a
MavenCompatibilityHelper . Approach based on
http://svn.apache.org/viewvc?view=revision&revision=r1461429 .
Fix #13 - Release a version compatible with Maven 3.1.0

Make aether-pi dependencies provided-scope.
cleanup dependencies
Merge pull request #15 from olamy/maven-3.1

Maven 3.1 support with dependency cleanup
Bump version to 2.1.0.
Add new contributors.
Update docs for Maven 3.1 and Mojo Executor 2.1.
Add Jelmer Kuperus to contributors.
[maven-release-plugin] prepare release mojo-executor-parent-2.1.0
[maven-release-plugin] prepare for next development iteration
Add "News" section to README.md.