Load .json from URL

[bhowes-tt]

Add the ability to load a .json file hosted somewhere else - like on dropbox or google drive, for example.

I tried this very quickly a while ago and ran into some security issues while testing with google drive. I think I got dropbox to work:
https://github.com/tt-acm/Spectacles.WebViewer/blob/gh-pages/js/SPECTACLES.js#L419-L445

Figure it out and add new UI, backup functions for loading remotely hosted .json files.

[bhowes-tt]

BTW I think it's fine if this only works from a couple of providers at first ... as long as we document how to host .json files on google and dropbox and load them with Spectacles, I think it's fine to start.

[bhowes-tt]

Note: we should test opening .spect files from dropbox before moving ahead with #20

[bhowes-tt]

Working pretty well. Dropbox and github work, google drive throws an error but does not sink the ship. Close enough for the alpha...