Logging

I suggest to use [logging](http://docs.python.org/3/library/logging.html) module to find some runtime bug easily. First it was inserted into some corner cases in `libearth.parser.rss2` module. If this PR has been accepted, I will find more places which needs to log.

To test logging correctly, [mock](https://pypi.python.org/pypi/mock) package was introduced in the testing environment.