Add a script to setup a development environment for EasyBuild

This script:
- clone all the EB repositories
- checkout the develop branch in eb-framework, eb-easyblocks and eb-easyconfigs
- create a module file to easily swith between this develop version of EB and any others

This is somehow related to this issue https://github.com/hpcugent/easybuild-framework/pull/730.
However, my script clones the git repositories instead of doing a simple installation. So it is easier to do commit, push and pull requests later on, and then contribute to EasyBuild.