[tools] Add ExpressionEvaluator class and eval_equation function.
[dice] Add back arithmetic expression functionality.

-Also allows for using many dice expressions in one command, just like before.
[module] Fix decorators in nickname_commands docstring
[Calc]Wolfram error output to be a little more userfriendly
[core] Guard against objects with no __name__.

- Fixes issue #313.
[rss] Report malformed XML; skip updating feed with unchanged etag

- Throw a debug message if the feed parser detects non-well-formed XML.
- Don't update the feed in the database if the etag and modified time
have not changed.