Correct comment
Ensure the long descriptions and formatting preserve unicode correctly

This is the first stage towards fixing issue319, at least
py.io.saferepr and py.code.ExceptionInfo need to be addressed as well.
Respect unicode in AssertionError argument

This is related to issue319
Add test for unicode assertion descriptions

Also clean up a few debugging leftovers.
add changelog: fix issue319 - correctly show unicode in assertion errors.  Many
thanks to Floris Bruynooghe for the complete PR.  Also means
we depend on py>=1.4.19 now.