Split protobuf into its own project

We don't want core to depend on protobuf.
diff --git a/core/build.gradle b/core/build.gradle
index 4b2e196..30bf385 100644
--- a/core/build.gradle
+++ b/core/build.gradle
@@ -5,8 +5,7 @@
 description = 'gRPC: Core'
 
 dependencies {
-    compile libraries.protobuf,
-            libraries.guava,
+    compile libraries.guava,
             libraries.jsr305
 }