Use json/pure, to avoid building

It's slower, but that's okay. I'd like to get this thing working on OSX
without a compiler installed. Whether or not that is actually feasible
remains to be seen.
Remove Mechanize dependency

The resulting code, based on Net::HTTP is rather uglier, but I think
less magical. Could do with more refactoring.
Implement alarms
Remove bin/icloud-reminders

This is replaced by the separate icloud-reminders-cli project.