Stop applying the java plugin automatically.

Require the user to apply either the java plugin or the android plugin
first, so that he has a choice between them.

Resolves #11

README.md

@@ -12,6 +12,12 @@ com.google.protobuf:protobuf-gradle-plugin:0.4.1 - Available on Maven Central.
 To use the protobuf plugin, include in your build script:
 
 ```groovy
+
+// For Java project, you must apply the java plugin first.
+apply plugin: 'java'
+// Or, for Android project, apply the Android plugin first.
+// apply plugin: 'com.android.application'
+
 apply plugin: 'com.google.protobuf'
 
 buildscript {

src/main/groovy/com/google/protobuf/gradle/ProtobufPlugin.groovy

@@ -67,7 +67,12 @@ class ProtobufPlugin implements Plugin<Project> {
             println("You are using Gradle ${project.gradle.gradleVersion}: This version of the protobuf plugin requires minimum Gradle version 2.2")
         }
 
-        project.apply plugin: 'java'
+        if (!project.plugins.hasPlugin('java')
+            && !project.plugins.hasPlugin('com.android.application')) {
+            throw new GradleException(
+                'Please apply the \'java\' plugin or the \'com.android.application\' plugin first')
+        }
+
         // Provides the osdetector extension
         project.apply plugin: 'osdetector'
 

testProject/build.gradle

@@ -7,7 +7,7 @@ buildscript {
     classpath "com.google.protobuf:protobuf-gradle-plugin:${rootProject.version}"
   }
 }
-
+apply plugin: 'java'
 apply plugin: 'com.google.protobuf'
 
 repositories {

testProjectCustomProtoDir/build.gradle

@@ -7,7 +7,7 @@ buildscript {
     classpath "com.google.protobuf:protobuf-gradle-plugin:${rootProject.version}"
   }
 }
-
+apply plugin: 'java'
 apply plugin: 'com.google.protobuf'
 
 repositories {