Simple base model

Added a simple base model that allow working with dataset with custom objects and simple methods to interact with the database.

Example:
```
class Weather(Model):
    _table = 'weather'
    _indexes = ['place', ]
    _fields = [
        ('date', datetime.now()),
        ('temperature', 0),
        ('place', None),
    ]
```

More info in the code and documentation.