impossible to query builders/n:builderid/builds?complete_at__gt=<unix_timestamp>

This is because the `valueFromString` method called from master/www/rest.py:310 is not implemented by `DateTime` class defined in /data/types.py:125.
```python
class DateTime(Instance):
name = "datetime"
types = (datetime.datetime)
ramlType = "date"
```

Start at master/www/rest.py:310 to trace the query parameter conversion, then inspect the DateTime class at data/types.py:125 and its valueFromString behavior. Confirm that builders/n:builderid/builds?complete_at__gt= can be queried successfully when the change is complete.

Written by the indexing model from the issue text.