RFE: Maven wrapper support

[vorburger]

@jmtd point re. "image variant... should it include Maven" in #56 remind me of another RFE:

This S2I builder image should let users build with a newer/older Maven version than the one included by default. The easiest way that I know of to enable this, that I know of, is to let the S2I script detect the presence of the Maven Wrapper (which fixes a version).

We can take (strong) inspiration from "prior art" worked on over here to implement this:

• https://github.com/fabric8io-images/s2i/pull/152/files