Remove excess flexibility in signature of helper method.
Add test showing that guards defined on the transition are called.
Add test showing that guards passed in are also called.
Document argument semantics for '_transition'.
Check both guards defined on the transition *and* guards passed in.

Fixes #8.
Changelog entry for #8.
Merge branch 'master' into 8-use_transition_guards
Merge branch 'master' into 8-use_transition_guards
Merge pull request #9 from repoze/8-use_transition_guards

Fix #8: include guards defined on transition when checking that it is allowed