Use a deque for timestamps
Move deque maxlens to class variables
Move request and socket exceptions to global variable
Add a default_update_interval variable

This is used as the update interval when a feed failed to download, it
has no timestamps, or the provided timestamps produced a negative item
interval.

In other words, when things get screwy.
Use __version__
Only hash feed objects on URL
Drop self.item_latest_timestamp
Remove unused instance variables
Update stale docstrings