Handle time edge-cases

[porglezomp]

There are two edge-cases that I figured out during discussion on #69 that need to be tested and fixed. From that discussion:

Exploring the semantics of the is_scheduled policies:

@ every # day(s) should wait at least # days. I think this should be independent of the times during those days, so if you read it at midnight on one day, and the morning a week later, less than 7 days have elapsed, but it should be considered as 7 days. I think this one is handled incorrectly, and we need to count days of the week rather than number of days in the elapsed time.

@ on monday/tuesday/etc should require that it was last read before that day, and it is now that day or later. Currently, if you last read it sunday night, and it's marked as @ on monday, I believe it will currently think 0 days have passed, and so not realize that you went from sunday -> monday.