Fixed naturaltime ignoring the months parameter + added test cases

I noticed that while the `naturaltime` function accepted a `months` parameter, it did not pass it on to the actual `naturaldate` function. I added the parameter and a few specific test cases.

    % python setup.py test                                                              
    running test
    ....
    test_naturalsize (tests.filesize.FilesizeTestCase) ... ok
    test_apnumber (tests.number.NumberTestCase) ... ok
    ...
    test_naturaltime (tests.time.TimeTestCase) ... ok
    test_naturaltime_nomonths (tests.time.TimeTestCase) ... ok
    ....    
    ----------------------------------------------------------------------
    Ran 13 tests in 0.013s
    
    OK