Added logging support using the standard python module.

Hi,

It seems that using standard python logging facilities is better way of storing runtime info. So I went ahead and added logging stuff to vitables. Basically now one can log using something like

```python
logger = logging.getLogger('vitables')
logger.info('Some message')
```
Logs will go to main gui logger.Logger object and optionally to a file specified on command line. Log level is set by providing different number of -v options.

1. Added command line options for selecting a log file and setting
a log level. ERROR level is used by default, -vvv selects DEBUG log level.

2. Updated logger.Logger class so that it can be used with
logging.StreamHandler

3. Set VTGUI.logger as stream handler for logging.

4. Added build directory to gitignore.

Alexey