fix rank filters for float structuring element
Create dummy WindowsError that is an actual Exception instead of None. The previous solution prevented installation on py3.3 and OSX 10.6.1.
Make sure non-zero values are treated as True
Add test case for different dtypes of structuring element
Merge pull request #483 from almarklein/windowserror

Create dummy WindowsError that is an actual Exception instead of None.
Merge pull request #1 from ahojnnes/thouis-master

Add test case for rank filter fix
Merge pull request #482 from thouis/master

fix rank filters for float structuring element