Add Loading Screen to GA Client

[DJSekora]

Calculations in the GA Client can take a while (multiply the time it takes to process a request in the Analyst Client, then multiply that by the size of the origin set), so it would be really nice to have some form of loading screen. The easiest solution might be to examine the JavaScript for the regular trip planning opentripplanner-webapp and figure out how the loading screen works there (this would also provide some consistency). However, after that it would also be great if we could add some indicator of the time remaining (might not be possible, depending on how the Java/JavaScript interaction works).