resolves #194 migrate project to multi-module Gradle build

Based on the excellent work by @ysb33r and advice by @aalmiray.

- add multi-module Gradle build
- remove Maven build
- setup Gradle wrapper
- relocate core to asciidoctorj-core subproject
- setup asciidoctorj-pdf and asciidoctorj-epub3 subprojects
- move bundling of asciidoctorj-epub3 into separate jar
- use JRuby Gradle plugin to manage gems
- activate and fix broken iconfont test
- only bundle essential files from RubyGems
- configure Gradle to generate Eclipse and IDEA project files
- create distribution for AsciidoctorJ (core)
- allow Gradle to generate start scripts for distribution
- add additional information to AsciidoctorJ manifest (dist only)
- configure distribution to bintray (not tested)
- autoload asciidoctor-pdf gem when backend is set to pdf
I'm not using that plugin yet. In fact, I didn't even realize it was a different plugin from the main JRuby one until a couple of hours ago.

Having said that, I have a feeling I'm still going to need to tweak it. At the very least, the data directory needs to be added by default, though as you can see some gems use that folder as a dumping ground & we are trying to include the minimal number of files. This will be a key learning experience.