Prepend imagen artifacts with "imagen".

[dromagnoli]

in jai-ext we had all the module's artifact names prepended by jt. that made it easy to quickly identify the JAI-EXT libraries.

In imagen we simply have the module name in the artifact name: This makes the imagen libraries less distinguishable within a WAR that includes all dependencies (in this screenshot, version 0.9.0).

Although that's not a big deal, I think it would be great to add an imagen- prefix so that all the imagen libs are easy to group and identify.