introduce maven-publish plugin, to publish jar files (spotbugs#97)

this change make it possible to publish spotbugs.jar and
spot-bugs-annotations.jar, with sources and javadoc.

build.gradle

@@ -1,5 +1,6 @@
 subprojects {
   apply plugin: 'java'
+  apply plugin: 'maven-publish'
   apply from: "$rootDir/gradle/eclipse.gradle"
   apply from: "$rootDir/gradle/idea.gradle"
   apply from: "$rootDir/gradle/test.gradle"

findbugs/build.gradle

@@ -255,6 +255,37 @@ task smokeTest {
   }
 }
 
+// https://docs.gradle.org/current/userguide/publishing_maven.html
+publishing {
+  publications {
+    mavenJava(MavenPublication) {
+      from components.java
+      artifactId 'spotbugs'
+
+      artifact sourcesJar {
+        classifier 'sources'
+      }
+
+      artifact javadocJar {
+        classifier 'javadoc'
+      }
+    }
+
+    annotations(MavenPublication) {
+      artifactId 'spotbugs-annotations'
+      artifact annotationsJar
+
+      artifact annotationSourcesJar {
+        classifier 'sources'
+      }
+
+      artifact annotationJavadocJar {
+        classifier 'javadoc'
+      }
+    }
+  }
+}
+
 // TODO : jsr305.jar (really?)
 // TODO : generatemanual (we should decide what to do with the manual)
 // TODO : generatepdfmanual