Use list for timestamps

A deque just isn't the right structure for this. Plus, intervals could
be incorrect if the feed items aren't posted in strict reverse
chronological order.
Store previous timestamp and item interval in JSON

Playing around with different sorting algorithms. These might be
useful (especially item interval).
Handle string timestamp in seconds_since
Use list for fingerprints
Add 'next_check' to JSON updates
Calculate factor automatically
Add a --strict option

This presents the feed updates in strict chronological order.
Move Index to Feed class variable
v0.3
Prepare for v0.3