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