Parse delimited from test

Hi, I need to parse multiple messages from one file. In Google protocol buffers I used [parseDelimetedFrom](https://developers.google.com/protocol-buffers/docs/reference/java/com/google/protobuf/AbstractParser#parseDelimitedFrom(java.io.InputStream)). This is my attempt to implement same method wire-way.
Ah, yeah. The problem is we buffer too much data and it gets left behind inside the `BufferedSource`. How very unfortunate. It seems we have no other choice but to take a `BufferedSource` as the parameter to avoid this.

cc @swankjesse