Add protoc wrapper script (#462)

This can be used as a drop-in replacement for the C++ binary protoc.
Use like this:

pip install grpcio-tools
nanopb/generator/protoc --help
diff --git a/generator/protoc b/generator/protoc
new file mode 100755
index 0000000..15f1d81
--- /dev/null
+++ b/generator/protoc
@@ -0,0 +1,8 @@
+#!/usr/bin/env python
+
+import sys
+from nanopb_generator import invoke_protoc
+
+if __name__ == '__main__':
+    status = invoke_protoc(['protoc'] + sys.argv[1:])
+    sys.exit(status)