Implement __del__ method to automatically close log files

This removes ResourceWarning expections on Python 3, such as:

  ResourceWarning: unclosed file <_io.TextIOWrapper name='twisted.test.test_logfile/LogFileTestCase/test_fromFullPath/3f20oo/temp/test.log' mode='w+' encoding='UTF-8'>
    result = f(*a, **kw)
Improve coverage of DailyLogFile.rotate()
Improve tests for DailyLogFile.getLog() + doc