Copy configurl.txt to .lantern.

Shameless copy&pasta from what we used to do with fallback.json.
First shot at having configurl.txt copied to user folder on install.

Untested as of yet.
Read configurl.txt and store the URL in settings.
Move existing logic to a separate class.

Also proof of concept to check that downloading the config works.
Parse S3 config.
Ignore unrecognized JSON properties.

So we can add them for the benefit of newer clients.
Hook DefaultProxyHandler to DefaultS3ConfigManager.java.

So it gets the fallback proxy info from there.  Also, it should know
to refresh its fallbacks when they change (this part is untested).
Keep rescheduling config recheck; logging improvements.

At this point I have also tested that reapplying configuration works.