Build -sources and -javadoc jars

Running `gradle build` within ethereumj-core will now produce -sources
and -javadoc jars in `build/libs`, alongside the already existing
classes jar. Run `gradle install` to install these artifacts to the
local `~/.m2` repository.

ethereumj-core/build.gradle

@@ -75,3 +75,18 @@ javadoc {
         "http://netty.io/4.0/api/"
     )
 }
+
+task sourcesJar(type: Jar, dependsOn: classes) {
+    classifier = 'sources'
+    from sourceSets.main.allSource
+}
+
+task javadocJar(type: Jar) {
+    classifier = "javadoc"
+    from javadoc
+}
+
+artifacts {
+    archives sourcesJar
+    archives javadocJar
+}