Support installation of artifacts to local maven repo

Apply Gradle's 'maven' plugin in order to support `./gradlew install`
task, similar to `mvn install` for building and copying artifacts to the
users local ~/.m2 repository.

ethereumj-core/README.md

@@ -32,6 +32,10 @@ Use IDEA 14 or better and import project based on Gradle sources.
 
 Note that in order to build the project without errors in IDEA, you will need to run `gradle antlr4` manually.
 
+#### Install artifacts into your local `~/.m2` repository
+
+Run `./gradlew install`.
+
 #### Publish ethereumj-core builds
 
 _TODO: integrate bintray gradle plugin_

ethereumj-core/build.gradle

@@ -2,6 +2,7 @@ plugins {
     id 'application'
     id 'eclipse'
     id 'jacoco'
+    id 'maven'
     id 'com.github.johnrengelman.shadow' version '1.2.0'
     id 'me.champeau.gradle.antlr4' version '0.1'
     id 'com.github.kt3k.coveralls' version '2.0.1x'
@@ -11,6 +12,7 @@ wrapper.gradleVersion = '2.2.1'
 
 sourceCompatibility = 1.7
 
+group = 'org.ethereum'
 version = '0.7.15-SNAPSHOT'
 
 mainClassName = 'org.ethereum.Start'