Add ability to cache data and upload to fitbit site

[qdot]

Just got an interesting request in. Someone wants to be able to take their fitbit to a place with spotty internet for a long period of time, and therefore cache off their data locally, then sync everything later.

Does seem like this would be a great idea for fitbit to implement themselves, but they don't seem to be doin' so well on the software side these days in their website alone, much less nifty features. :)