Live feed data

[alukach]

When experimenting with the ability to pre-load data from a GET parameter (#139), I thought it was pretty interesting to point DropChop at live data sources (such as the USGS dataset of earthquakes within the hour). This sparked the idea of having DropChop poll a supplied data feed and update it as new data arrives. This would likely go in the Add menu and prompt the user with some related options:

• poll_frequency: Number, how often (in seconds?) to poll the feed.
• append: Boolean, whether to add newly discovered data to existing data or to replace data with every poll.

Assuming that #141 has been implemented, any analysis on the feed would update as the feed updates.

Some questions:

• It would be best if the feed could be added with a GET param like url and gist offers (feed makes sense), but I'm unsure how someone would add the feed options via the GET param.
• If append is true, how do we diff the data? Probably compare the layer's JSON value to the new data, although we'd want to make sure that things like styling data wouldn't interfere with the diff.
• Assuming that Style layers in layerlist #126 is complete, how does a layer's style carry over to the new data? Maybe style should be stored on the layer like described in Store / edit operation settings  #140.