[MEXEC-135] add ability to invoke an external environmental script

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/exec-maven-plugin@19738 52ab4f32-60fc-0310-b215-8acea882cd1b
[MEXEC-135] add ability to invoke an external environmental script

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/exec-maven-plugin@19743 52ab4f32-60fc-0310-b215-8acea882cd1b
[MEXEC-137] add IT mexec-137, apply patch provided by Piet van Dongen. However, the patch is disable since the test is happy without the patch.  Need further investigation

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/exec-maven-plugin@19751 52ab4f32-60fc-0310-b215-8acea882cd1b