tests: Fix failing unittest test_watchdog_observers_polling.TestPollingEmitter

There was both a FileDeletedEvent and a DirDeletedEvent for the entry, just
keep the latter. Also add missing to_file entry.

This makes the test work on OS X (tested on 10.8.3) and Linux (tested on Ubuntu 12.10)
Merge pull request #152 from Ormod/polling_test_fix

tests: Fix failing unittest test_watchdog_observers_polling.TestPollingE...