Added a debug feature for the Database class
I'm not sure I understand these changes.
 http://peewee.readthedocs.org/en/latest/peewee/cookbook.html#logging-queries
 The developer is not able to choose whether or not they want to disable the logger providing debug output. You gave a solution here, https://github.com/coleifer/peewee/issues/254, to write

```
import logging
logger = logging.getLogger('peewee')
logger.addHandler(logging.NullHandler())
```

but the person who raised this issue stated that the code provided did not work. When I added these lines to my code base, it did not fix the logger problem.

I think it would be favorable to enable logging while instantiating the database object by changing the debug keyword argument, which I added.

When I first came across the log message being printed into the console, I thought there was an error with my code, but it was just because logging is not disabled by default.
 > You might try then peewee_logger.handlers = []. The logging docs are pretty helpful.

From my final comment in the issue you referenced.
 The log framework has multiple log levels so you can subscribe to the ones you care about. Adding another layer of API in-between seems redundant, so I'm going to close this.

I changed the Database class so that you can choose to enable logging. For example,

```
db = SqliteDatabase('data.sqlite3', debug=True)
```

will enable the logger to log messages onto the console while the python interpreter is running.
