WIP: doc updates
Removes max-width rule, which cuts off background for git clone line

The git clone line on the download page, that is.
WIP: linkifies xpi
Update for Mozilla SDK 1.15, s:require('timers'):require('sdk/timers'):
Small documentation changes
Removes JSON.stringify implementation

JSON is implemented natively in pretty much every browser these days:
http://caniuse.com/#search=JSON

That link only shows the five most recent versions of Firefox and
Chrome.  Both have implemented JSON going much further back than Firefox
25 or Chrome 31.
Updates documentation of methods in prelude.js

- Adds more examples
- Clarifies some descriptions
- Fixes some outdated documentation
- Formatting changes
Updates more documentation
Updates headless script, documentation, and basicRun example