Document binary format and usage implied by the binary protobuf writer

[sakrejda]

The pull request was fully tested but now it only makes sense to me:

• [x] add documentation to write_delimited_pb function, including comments on how this defines a binary file/streaming format.
• [x] add documentation to read_delimited_pb function
• [x] add usage example on how to use read_delimited_pb function to read the file/stream format.
• [x] add comments to tests for each writer component
• [x] add comments on the writer itself, for each method
• [x] add a usage example to the writer itself.

[sakrejda]

Doing code cleanup separately from doc.