addes support for lambda functions on model data

Hey, 

I added support for lambda functions that is applied on the data field. Here is an example, how it works:
The model definition:
```python
class Events(models.Model):
    key = models.CharField(max_length=255,unique_for_date="date",db_index=True)
    date = models.DateField(db_index=True)
    clicks = models.PositiveIntegerField(default=0)
```
view code:
```python
    eventdata = \
        DataPool(
           series=
            [{'options': {
               'source': Events.objects.filter(key=key)},
              'terms': [
                ('date',lambda d: d.toordinal()),
                'clicks'
                ]}
             ])

    #Step 2: Create the Chart object
    cht = Chart(
            datasource = eventdata,
            series_options =
              [{'options':{
                  'type': 'line',
                  'stacking': False},
                'terms':{
                  'date': [
                    'clicks']
                  }}],
            chart_options =
              {'title': {
                   'text': 'Clicks'},
               'xAxis': {
                    'title': {
                       'text': 'Date'}}},
            x_sortf_mapf_mts = (None,lambda i: datetime.fromordinal(i).strftime("%d-%m"),False)
```