MRG: fixing the failing canica test 

Attempt to solve issue #60. 

Made the test_canica test more robust by replication of ICA with different random seeds and selection of the seemingly best decomposition. 
Then decreased the test image size for the sake of computation time.
 
Note that the example may still fail, though with lower probability. I'm not satisfied by this fix, but it's the best one I've been able to run with some success.

Actually, this issue is excellent to understand why ICA sucks.