Explicitly specify protoc plugin in Plugin test target

This change explicitly specifies the swift grpc ptotoc plugin in the
`Plugin` directory, so users without `.` in the path can run the test
target.

Also add some build artifacts from the Plugin directory to .gitignore.

.gitignore

@@ -3,3 +3,5 @@ project.xcworkspace
 xcuserdata
 .build
 third_party/**
+Plugin/Packages/**
+Plugin/protoc-*

Plugin/Makefile

@@ -9,7 +9,7 @@ build:  clear
 	cp .build/debug/protoc-gen-swift .
 
 test:	build
-	protoc ../Examples/Echo/echo.proto --proto_path=../Examples/Echo --swiftgrpc_out=. 
+	protoc ../Examples/Echo/echo.proto --proto_path=../Examples/Echo --plugin=./protoc-gen-swiftgrpc --swiftgrpc_out=. 
 	diff echo.client.pb.swift ../Examples/Echo/Generated/echo.client.pb.swift
 	diff echo.server.pb.swift ../Examples/Echo/Generated/echo.server.pb.swift