Restore M2 execution, ${plugin} is only available for M3

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/jdiff-maven-plugin@17991 52ab4f32-60fc-0310-b215-8acea882cd1b
Add animal sniffer to prevent JDK6 leaks

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/jdiff-maven-plugin@17992 52ab4f32-60fc-0310-b215-8acea882cd1b
replace jdk6' String.isEmpty() with StringUtils.isEmpty()

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/jdiff-maven-plugin@17993 52ab4f32-60fc-0310-b215-8acea882cd1b
change Maven prerequisite to 2.2.1

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/jdiff-maven-plugin@17994 52ab4f32-60fc-0310-b215-8acea882cd1b
More 2.2.1 fixes

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/jdiff-maven-plugin@17995 52ab4f32-60fc-0310-b215-8acea882cd1b
Update integration tests for MJDIFF-13

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/jdiff-maven-plugin@17998 52ab4f32-60fc-0310-b215-8acea882cd1b