Document the codebase

[kennib]

Currently the broad overview of how the code works is in the mind of @kennib.
It needs to be written down in a more rigorous manner.

[kennib]

Issue #49 (refactoring) will hopefully make this issue a simpler task.

[techieshark]

Was looking into elm doc stuff today. Findings (just an fyi and putting all these links in one place):

• Elm has a code documentation format specified: http://package.elm-lang.org/help/documentation-format
• and this codebase is somewhat minimally documented, as alluded to in this issue description.
• and if you run elm-make --docs=documentation.json you get a file that you can stick in http://package.elm-lang.org/help/docs-preview to preview the docs in a browser rather than in a code editor.
• but there's apparently no way to generate local docs otherwise, despite being requested by others Elm users (Feature request: preview elm-doc document comment locally elm/elm-lang.org#169). I've started looking at generating a docset for Dash: Possible to generate docs for unpublished code? pdamoc/elm-docset#9 as one solution.