Move common protobuf .protos to common area under Examples/Google, improve setup scripts.

Examples/Google/Datastore/RUNME

@@ -7,7 +7,9 @@
 # protoc-gen-swiftgrpc binaries and assumes that protoc-gen-swift 
 # is installed in $HOME/local/bin.
 
-git clone http://github.com/googleapis/googleapis
+if [ ! -d "googleapis" ]; then
+  git clone http://github.com/googleapis/googleapis
+fi
 
 protoc \
  	googleapis/google/datastore/v1/datastore.proto \

Examples/Google/NaturalLanguage/RUNME

@@ -7,7 +7,9 @@
 # protoc-gen-swiftgrpc binaries and assumes that protoc-gen-swift 
 # is installed in $HOME/local/bin.
 
-git clone http://github.com/googleapis/googleapis
+if [ ! -d "googleapis" ]; then
+  git clone http://github.com/googleapis/googleapis
+fi
 
 protoc \
  	google/cloud/language/v1/language_service.proto \
@@ -15,7 +17,7 @@ protoc \
 	google/api/http.proto \
 	google/protobuf/descriptor.proto \
  	-Igoogleapis \
-	-Iprotoc/include \
+	-I../common/include \
 	--swift_out=googleapis \
 	--swiftgrpc_out=googleapis