Added tests for generators, settings, and pelican_import

I wrote a few tests to improve Pelican's code coverage and begin to better understand the codebase. I'm not entirely sure if I've written these tests in an adequate manner, so I might need a little bit of guidance. Also, I noticed that some tests contained docstrings with a description of the test whereas others simply have commented-out descriptions. The benefit of a descriptive docstring is that the description is printed to the output of `nose` (rather than just the name of the test function); is this something that we would want to standardize?