rsolve: return None if it's not able to find solutions
Fix broken link to aboutus.html
test_kane2.py: remove deprecated (see issue 3381) usage of zeros((n, m))
Fix return values in test_recurr.py, rsolve_hyper: 0 -> None
Merge pull request #1874 from skirpichev/misc

Fix return value for rsolve + misc fixes