[#734] Make the picocli jar OSGi friendly

* added the bnd plugin to the build
* defined a bnd.bnd file to handle the package export
(fixes #734)

bnd.bnd

@@ -0,0 +1,3 @@
+Import-Package: !org.fusesource.jansi, *
+Export-Package: picocli
+-fixupmessages "^Classes found in the wrong directory: \\{META-INF/versions/9/module-info\\.class=module-info}$"

build.gradle

@@ -1,5 +1,7 @@
 import java.nio.file.Files
 import java.nio.file.Paths
+import aQute.bnd.gradle.Bundle
+import aQute.bnd.gradle.Baseline
 
 group 'info.picocli'
 description 'Java command line parser with both an annotations API and a programmatic API. Usage help with ANSI styles and colors. Autocomplete. Nested subcommands. Easily included as source to avoid adding a dependency.'
@@ -12,6 +14,7 @@ buildscript {
         maven {
             url "https://plugins.gradle.org/m2/"
         }
+        mavenCentral()
     }
 
     dependencies {
@@ -19,6 +22,7 @@ buildscript {
         classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.15'
         classpath "com.jfrog.bintray.gradle:gradle-bintray-plugin:$gradleBintrayPluginVersion"
         classpath "gradle.plugin.org.beryx:badass-jar:1.1.3"
+        classpath 'biz.aQute.bnd:biz.aQute.bnd.gradle:4.2.0'
     }
 }
 
@@ -27,6 +31,7 @@ apply plugin: 'distribution'
 apply plugin: 'maven-publish'
 apply plugin: 'com.jfrog.bintray'
 apply plugin: "org.beryx.jar"
+apply plugin: 'biz.aQute.bnd.builder'
 
 
 allprojects {
@@ -358,6 +363,28 @@ publishing {
         }
     }
 }
+apply plugin: 'java'
+
+configurations {
+    bundleCompile
+    baseline
+}
+dependencies {
+    baseline('group': group, 'name': jar.baseName, 'version': "(,${jar.version}[") {
+    force = true
+    transitive = false
+  }
+}
+
+sourceSets {
+    bundle
+}
+task bundle(type: Bundle) {
+    from sourceSets.bundle.output
+    bndfile = 'bnd.bnd'
+    sourceSet = sourceSets.bundle
+}
+
 /*
 Release procedure:
 1. edit version numbers: remove -SNAPSHOT classifier