Removes Candidate.is(_any)_active(...).

Instead of is_active(...) use: get_category(...) in (u"walk", u"stumble")

There is no replacement for is_any_active(...).  Instead the states of
a candidate in other communities no longer influenc each other.
Using dispersy_yield_verified_candidates instead of manual filtering.
Add documetation.
Merge remote-tracking branch 'remotes/upstream/devel' into candidate-cleanup

Conflicts:
	dispersy.py
Merge pull request #80 from boudewijn-tribler/candidate-cleanup

Removes Candidate.is(_any)_active(...).