JavaDocs served via GitHub pages

Gradle plugin to automatically publish JavaDocs to GitHub pages when `githubPages` goal is run.

See https://github.com/ajoberstar/gradle-git#travis-ci-authentication-notes for setting the GH_TOKEN variable, I can't do that bit.

@sk89q, I believe you setup the .travis file so it makes sense for you to do that bit I guess.

SpongePowered/Meta#5
Ah, just seen the official page earlier, I must have just scanned over it because it seems the artifact has a different name for 0.10.0, not doing very well today it seems!