use cv2.mean & add automatic conversions

opencv_functions.py : use cv2.mean() instead of numpy.average()

some tests failed due to passing in a nested list instead of a numpy
array, so I added automatic conversions:
=> convert_data() added in convert.py to handle automatic conversion
between nested list <-> numpy array <-> CvMat <-> IplImage
=> each connector now has an optional data_types attribute which is a
list of the valid data types
=> Connector.set_value() now first passes the new value to
convert_data() to do automatic data type conversion
unittests for convert_data()

Added unittests for all useful automatic datatype conversions
Import convert_data() into the improcflow.logic namespace
small item added to test-TODO
move tests to separate folder
split test_all.py in 3 files

0) TODO.txt : keep track of tests / features that I need to add
1) test_flow_logic.py : test the main dataflow logic
2) test_automatic_conversion.py : test the automatic conversion between
ndarray / iplimage / cvmat