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
That's cool the only problem I see is that I think these changes should be
done forking my repo because I have already worked on splitting project and
creating some standard classes to be used in jruby, nashorn, rhino, ....
implementations. Well I will migrate them in upstream repo.

Also I see that pom is not updated, so now we are 100% Gradle right?

El Sat Nov 29 2014 at 1:53:33 PM, Dan Allen (<notifications@github.com>) va
escriure:

> In build.gradle:
>
> > +  javadoc {
> > +    failOnError = false
> > +  }
> > +
> > +  task sourcesJar(type: Jar, dependsOn: classes) {
> > +    from sourceSets.main.allSource
> > +    classifier = 'sources'
> > +  }
> > +
> > +  task javadocJar(type: Jar, dependsOn: javadoc) {
> > +    from javadoc.destinationDir
> > +    classifier 'javadoc'
> > +  }
> > +
> > +  apply plugin: 'com.github.jruby-gradle.base'
> > +  //apply plugin: 'com.github.jruby-gradle.jar'
>
> I added that line when I finally found the plugin as a reminder to use it.
> I was on to other tasks at that point and didn't want to mess with what I
> had working.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/asciidoctor/asciidoctorj/pull/222/files#r21053688>.
>