Added value transformations for columns

Columns now might define a value transformation function that will transform the value before returing it. Given the following column:

```python
{
    'name': 'price',
    'required': True,
    'validator': is_number,
    'transform': lambda x: Decimal(x)
}
```

and reading a standard CSV file with a column similar to: `iPhone 5c,USD,799.99,`

```python
reader = smartcsv.reader(csv_file, columns=columns)
my_object = next(reader)
print(my_object)  # will return Decimal("799.99")
```