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