Resume archived entries if no current entries

Hey,

I noticed [Ruby5 mentioned timetrap today](http://ruby5.envylabs.com/episodes/459-episode-423-december-3rd-2013), congrats! Although they say it's brand new.

Anyway, I added this because quite often I archive a sheet's entries (e.g. after I bill a client for them) while I am still working on the same feature. This just allows me to `t r` even if the last entry has been archived.

Toby