Add a script to setup the 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 PR was initially here https://github.com/hpcugent/easybuild/pull/19.

This is somehow related to this issue hpcugent/easybuild-framework#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.