resolves #194 migrate project to multi-module Gradle build

- add multi-module Gradle build
- remove Maven build script (pom.xml)
- setup Gradle wrapper
- relocate core to asciidoctorj-core subproject
- setup asciidoctorj-epub3, asciidoctorj-pdf and asciidoctorj-distribution subprojects
- move bundling of asciidoctorj-epub3 into separate jar
- define version and properName properties for each artifact in gradle.properties
- use JRuby Gradle plugin to manage gems
- bundle only essential sources from gems; copy to output folder of main source set
- configure Gradle to generate Eclipse and IDEA project files
- create distribution zip for AsciidoctorJ (bundles core, epub3 and pdf)
- use Gradle to generate start scripts for distribution
- optimize JVM flags for start scripts in distribution
- add additional information to manifest of artifacts
- configure install to local Maven repository
- configure signing (GPG) plugin
- configure upload to Bintray (creates package & version; includes distribution zip)
- update script to test against Asciidoctor upstream (using Maven to install gem)
- activate and fix broken iconfont test
- autoload asciidoctor-pdf gem when backend is set to pdf
sync README content with website; add info about Gradle build

- sync content from website
- add information about Gradle build
- explain how to perform a release