Repo lib builder

Initial version of the tool for creating repository libraries. Based on EditorDemoBuilder.
The tool is created as a new project (repo-builder). I discarded the idea of using a different repository - makes development much harder and benefits are unclear.
It uses ImageMagick for image conversion and fast thumbnail creation. Works well, although took a while to get the best configuration options so Libgdx does not cry out loud. ImageMagick needs to be installed locally. 

You can see https://github.com/jtorrente/ead/blob/repo-lib-builder-2/repo-builder/src/main/java/es/eucm/ead/repobuilder/libs/Example.java for an example on how it works.

Also you can download an executable jar file from https://github.com/e-ucm/ead/wiki/Common-maven-recipes if you just want to run it