added the OutputDirectory output type

This pull request adds a new output type to Sextante: OutputDirectory. It enables using directories as outputs when building Sextante GeoAlgorithms.

It is marked as Feature #6055 in the issue tracker[1].

Example usage:

```python
from sextante.outputs.OutputDirectory import OutputDirectory

class TestAlgorithm(GeoAlgorithm):
    
    def defineCharacteristics(self):
        self.addOutput(OutputDirectory('OUTPUT_DIR', 'Choose an output directory for saving files'))
```

[1] - http://hub.qgis.org/projects/sextante/issues