Polishing Makefile to better install targets.

-) renamed cpp_plugin and ruby_plugin to grpc_cpp_plugin and grpc_ruby_plugin.
-) installing plugins.
-) install will now run protobuf's installation too.
-) adding documentation about installation prefix.
diff --git a/build.json b/build.json
index c552228..c7c640d 100644
--- a/build.json
+++ b/build.json
@@ -1575,7 +1575,7 @@
       ]
     },
     {
-      "name": "cpp_plugin",
+      "name": "grpc_cpp_plugin",
       "build": "protoc",
       "language": "c++",
       "headers": [
@@ -1747,7 +1747,7 @@
       ]
     },
     {
-      "name": "ruby_plugin",
+      "name": "grpc_ruby_plugin",
       "build": "protoc",
       "language": "c++",
       "src": [