Fix import error (when running on linux: "python qgis2img")

Otherwise qgis2img fails to load:

martin@zenbook:~/qgis/qgis2img$ python qgis2img bench --types project /data/gis/x.qgs
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/martin/qgis/qgis2img/qgis2img/__main__.py", line 3, in <module>
    import qgis2img.render
ImportError: No module named qgis2img.render