Make general changes to the gradle setup

I have added various improvements to the gradle setup.

- I separated the mod version, Minecraft version and forge version into `gradle.properties`. This helps separate the commonly updated values from `build.gradle`.
- I did some substantial edits to the `README.md` in  relation to the changes I made, also some general cleaning.
- I added a sources and deobf jar to the gradle setup.
- I made some small style fixes in `build.gradle`. I'm not too sure about this so I can remove this if you want.

I can also reduce this to 1 commit if you want, but I think these commits contain logical steps although I could condense it more if needed.

Please actually read this instead of blind merging.