Inline doc

Inline documentation, refs #1048. Adds markdown-formatted help files under data/doc and pulls them together with `build.js`, and adds a UI element with a new button on the left and a new slide in/out pane for showing help info.

TODO (not necessarily before merge)

* Needs an icon for the help pane @samanpwbb 
* UI rework - the Table of Contents kind of sucks atm
* Ideas on how we translate this @ansis 
* Needs more docs - rivers, points, how to use imagery etc.
* How to manage help-related images / screenshots. In flickr? In the repo?

More Docs:

- Roads
- Buildings
- Points of interest
- Water bodies
- Parks
- Addresses
- Turn restrictions
- GPS (pending #277)