Added tests for a couple of errors which may be raised by Aggregator#parse_feed.
Added tests for a couple of errors which may be raised by Aggregator#parse_feed.

They're crude tests and the test strategy needs some work, but test coverage is
now over 85% (C0, via rcov). Not yet being tested: email configuration and error
reporting, feed truncation and sorting, correct aggregation of multiple feeds.

svenaas/audrey2