Add nexus-staging plugin

Author: lkorth

CardForm/build.gradle

@@ -1,6 +1,7 @@
 apply plugin: 'com.android.library'
 apply plugin: 'maven'
 apply plugin: 'signing'
+apply plugin: 'io.codearte.nexus-staging'
 
 android {
     compileSdkVersion 22
@@ -54,6 +55,10 @@ signing {
     sign configurations.archives
 }
 
+nexusStaging {
+    packageGroup = "com.braintreepayments"
+}
+
 def sonatypeUsername = System.properties.containsKey('sonatypeUsername') ? System.properties['sonatypeUsername'] : ''
 def sonatypePassword = System.properties.containsKey('sonatypePassword') ? System.properties['sonatypePassword'] : ''