Refactor log.py and change the default logging behavior

Previously, logging messages were written to "tuf.log" *and* and the console, by default.  Modules had to explicitly disable the logger to silence console messages.  TUF, when integrated by a software updater, should not log messages to console by default.  The design change now forces modules to call tuf.log.add_console_handler() to enable logging messages to console.  The logger, file, and console handlers may have independent logging levels.