[rss] Fix error looking for timestamp on new RSS items; add date format

Was throwing an AttributeError on items without an 'updated' value.

Added DATEFORMAT as a variable at the top of the module; this could
possibly be added as a configuration option.
Add timezone link to unknown user/timezone message.
[bot.py] Handle a 0-length string properly

Closes #396
Merge pull request #395 from saltire/master

[rss] Fix error looking for timestamp on new RSS items; add date format
[Clock]Forgot a ), Thanks miggyb
[weather] Better sanity checking of Yahoo data

Closes issue #397
Closes issue #372