✨ Gradle support

[jmle]

👀 See Gradle support enhancement

(#598) Adds support for fetching dependencies with Gradle projects.
(#599) Adds support for downloading dependency sources.

• This implementation assumes that there is a Gradle wrapper included in the project, otherwise it is not possible to know which Gradle version is needed.
• To maximize compatibility with most Gradle versions, JDK8 must be used. There are a number of ways to specify this on newer Gradle versions, but for older versions the only thing that seems to work is setting JAVA_HOME.

❗ This PR supersedes #591

🟢 Tested against https://github.com/andyjduncan/gradle-example and https://github.com/kissaten/gradle-multi-project-example