Write more docstrings and comments

[felipedau]

This has been requested by @david415. unMessage initially had more docstrings and comments, but that habit got lost :/ Some ideas for this:

• I think that #66 should be finished first, because the public API will change
• Comment what is necessary - good code is mostly self explanatory
• If a comment is long, simplify the code
• Ask someone that is not familiar to the code to read it - improve based on their feedback
• Create docs for the public API (this can also be a separate issue)