Concurrency + Polling HTTP requests

[JhnBrunelle]

We need to implement a polling system to fetch rover data changes. Maybe hold off on this until we break up the app into multiple controllers so we can do this multithreading. Also should utilize inheritance, so we don't need to rewrite the polling function for each.

Should also be able to set the polling rate

[JhnBrunelle]

Made progress, will implement now

[joshkarbi]

May be nice to have fetchImage and fetchText API's.

This would only be good though if we could avoid double-creating FX Image objects