[EXT] Release 0.0.13
Updated the main Rakefile

* Removed the shorcut tasks
* Improved documentation and behaviour for the `build` and `status` tasks
* Added a `builds` task
[GEM] [CLIENT] Stop the testing cluster in `shutdown` only when it's running
[EXT] Display any exception encountered when stopping the test cluster nodes
Added a note about version compatibility to READMEs

Gem 0.4.x => Elasticsearch 0.90
Gem 1.x   => Elasticsearch 1.0
[GEM] Added note about compatibility into the main gem README
[DOC] Clarified Elasticsearch compatibility for regular and pre-release versions