Added the `setup` Rake task
[CI] Used `rake setup` in the Travis configuration
Changed the way elasticsearch/elasticsearch repository is embedded

Instead of using a submodule, do a regular Git clone in the `tmp` directory.

Changed the `rake setup` task to do a clone, if the repository doesn't exist yet.

Updated all the other tasks.
Added chapter about development to the main README
Added chapter about development to the subproject's READMEs