Send JSON-encoded posts instead of form-encoded.

Micropub form-encoded requests can't have nested objects like ate[type]=[h-food]

closes #8

adds screenshot for docs page

alexa seems to sometimes send other values

probably rough around the edges still

posts photos to the media endpoint if set

disable appcache

* stores location prefs client side
* retrieves options immediately if location checkbox is unchecked
* if location fails, loads buttons w/o location context
* waits for location to succeed to load options
* if no location is provided, uses location of last entry when retrieving buttons