Inline Field Sorting

Enable sorting by clicking on header heads.

## Example use
```python
class ExampleTable(tables.Table):
    name = column.Column(sortable=True, sort_field='full_name')
    total = column.Column(header='Total', accessor='count_all', css_class='ralign', sortable=True)

    table_sequence = ['name', 'total']
```

## Visual
### Unsorted
both options are shown
![unsorted](http://f.cl.ly/items/1N3I2w3o3f0f0N3t1A2v/Screen%20Shot%202013-11-19%20at%204.42.52%20PM.png)

### Sorted Desc
the opposite option is shown and clickable to change the sort.
![sorted](http://f.cl.ly/items/2a2J332R2K3a3U031m0/Screen%20Shot%202013-11-19%20at%204.43.10%20PM.png)