Added an exec resource to download the Gradle .zip file.
Added an exec resource to unzip the Gradle .zip file.
Added a before => so that the download always runs before the unzip.
Added an exec resource to move Gradle out of the version-numbered folder.
Added exec resources to export GRADLE_HOME and a PATH that has Gradle's bin/ folder in it.
Merge branch 'feature/add_execs' into develop
Added manifest doc about the version parameter.
Added a usage example to the README.