Add the gradle wrapper and cleanup build.gradle

[Su5eD]

The issue

As explained in issue #27, the gradle wrapper is missing from the source code, and several dependencies are unresolvable.

Changes made

• Added the gradle wrapper version 4.9
• Added a .gitignore to prevent unnecessary files from being commited
• Moved all version constants to a separate file gradle.properties for easier changing
• Improved build.gradle
  • Cleaned up inconsistent indentation to 2 spaces
  • Declared repositories for IC2, TechReborn, and RebornCore