Increase control over logging

I would like to have more control over logging, this ticket try to do that.

I had to drop the support to --log level, because I wasn't able to found a way to change on runtime the level of logging once is read from configuration

Eg:

```php
$ python2.7 dynamodump.py -m backup -r local --host 127.0.0.1 --port 4567 --accessKey key --secretKey secret -s foobars 
[2014-12-02 01:26:34,880] INFO: Starting backup for foobars..
[2014-12-02 01:26:34,880] INFO: Dumping table schema for foobars
[2014-12-02 01:26:34,884] INFO: Dumping table items for foobars
[2014-12-02 01:26:34,886] INFO: Backup for foobars table completed. Time taken: 0:00:00
```