Support testing using QGIS > 2.0

Tested using QGIS 2.0, QGIS 2.2, and QGIS master (dce4a77) - It can find the right module

Notes: 
- Using QGIS 2.2, on ```safe_qgis``` the tests that are failing:
  - ```test_check_postprocessing_layers_visibility: safe_qgis.impact_statistics.test.test_postprocessor_manager.py: 107```
  - ```test_issue306: safe_qgis.widgets.test.test_dock.py: 782```
  - ```test_newLayersShowInCanvas: safe_qgis.widgets.test.test_dock.py: 1156```
- Using QGIS Master (dce4a77), on ```safe_qgis```:
  - ```test_check_postprocessing_layers_visibility: safe_qgis.impact_statistics.test.test_postprocessor_manager.py: 107```
  - ```test_issue306: safe_qgis.widgets.test.test_dock.py: 782```
  - ```test_newLayersShowInCanvas: safe_qgis.widgets.test.test_dock.py: 1156```