Create library (core, VR, AR) artifacts on Maven

[codeanticode]

After shutdown of JCentral, we need a new repository to release the core, VR, and AR libraries as artifacts to include in other projects through Gradle, etc. Some options:

• Maven repo (The traditional / standard option) http://maven.apache.org/guides/mini/guide-central-repository-upload.html
• Self hosted https://stuartsierra.com/2009/09/08/run-your-own-maven-repository
• using Github (not that nice apparently) http://stackoverflow.com/questions/14013644/hosting-a-maven-repository-on-github
• https://jitpack.io is quite an easy option

Maven seems the best option, trying to automate the creation/upload of maven artifacts in this branch:

https://github.com/processing/processing-android/tree/maven-publishing