Workaround for Maven-2 compatibility.

* Use String parameters and Enum.valueOf methods because old versions of
  plexus-container-default used in Maven-2 do not include a EnumConverter.
* Add implementation hints in POMs because Maven-2 is unable to guess
  implementing classes correctly.
  See http://blog.sonatype.com/people/2011/03/configuring-plugin-goals-in-maven-3/#.UpkF45FIhsp)
* Use concrete type ArrayList instead of List in MergeMojo because Maven-2 is not
  able to deduce a corresponding class for an interface.
Update documentation of CheckMojo and explain the need for an implmentation hint with Maven-2
Force source and target in examples to at least JDK5.

* This is needed when you want to compile Jacoco with Maven 2.2.1
  because the default maven-compiler-plugin version here assumes
  JDK1.3 as default.
* With Maven 3 the default JDK is 1.5 so buildhive did not detect
  this.