Migrate from PARSER to parser() as a way of getting the parser of a protobuf message.

Upgrade to protobuf-3.0.0-alpha-3.1 that adds parser(). PARSER will go away eventually.
diff --git a/README.md b/README.md
index 40645c3..c8e4413 100644
--- a/README.md
+++ b/README.md
@@ -63,7 +63,7 @@
       <artifactId>maven-protoc-plugin</artifactId>
       <version>0.4.2</version>
       <configuration>
-        <protocArtifact>com.google.protobuf:protoc:3.0.0-alpha-3:exe:${os.detected.classifier}</protocArtifact>
+        <protocArtifact>com.google.protobuf:protoc:3.0.0-alpha-3.1:exe:${os.detected.classifier}</protocArtifact>
         <pluginId>grpc-java</pluginId>
         <pluginArtifact>io.grpc:protoc-gen-grpc-java:0.7.1:exe:${os.detected.classifier}</pluginArtifact>
       </configuration>
@@ -106,7 +106,7 @@
   }
 }
 
-protocDep = "com.google.protobuf:protoc:3.0.0-alpha-3"
+protocDep = "com.google.protobuf:protoc:3.0.0-alpha-3.1"
 protobufNativeCodeGenPluginDeps = ["grpc:io.grpc:protoc-gen-grpc-java:0.7.1"]
 ```
 
@@ -150,13 +150,13 @@
 generation. Most users only need to use `skipCodegen=true` as discussed above.
 
 ### Build Protobuf
-The codegen plugin is C++ code and requires protobuf 3.0.0-alpha-3.
+The codegen plugin is C++ code and requires protobuf 3.0.0-alpha-3.1.
 
 For Linux, Mac and MinGW:
 ```
 $ git clone https://github.com/google/protobuf.git
 $ cd protobuf
-$ git checkout v3.0.0-alpha-3
+$ git checkout v3.0.0-alpha-3.1
 $ ./autogen.sh
 $ ./configure
 $ make
@@ -195,15 +195,15 @@
 Gradle to find protobuf:
 ```
 .\gradlew install ^
-    -PvcProtobufInclude=C:\path\to\protobuf-3.0.0-alpha-3\src ^
-    -PvcProtobufLibs=C:\path\to\protobuf-3.0.0-alpha-3\vsprojects\Release
+    -PvcProtobufInclude=C:\path\to\protobuf-3.0.0-alpha-3.1\src ^
+    -PvcProtobufLibs=C:\path\to\protobuf-3.0.0-alpha-3.1\vsprojects\Release
 ```
 
 Since specifying those properties every build is bothersome, you can instead
 create ``<project-root>\gradle.properties`` with contents like:
 ```
-vcProtobufInclude=C:\\path\\to\\protobuf-3.0.0-alpha-3\\src
-vcProtobufLibs=C:\\path\\to\\protobuf-3.0.0-alpha-3\\vsprojects\\Release
+vcProtobufInclude=C:\\path\\to\\protobuf-3.0.0-alpha-3.1\\src
+vcProtobufLibs=C:\\path\\to\\protobuf-3.0.0-alpha-3.1\\vsprojects\\Release
 ```
 
 The build script will build the codegen for the same architecture as the Java