adds setting callback specs, also converts old setting tests to specs
implements callbacks for settings
dry up setting specs
fire callbacks only when setting was stored sucessfully
adds a spec to test invocation of multiple callbacks
specs for the OP::Notifications based settings notifier:
* arguments to the block is a hash
* no clearing of callbacks
* no callback objects
based the settings callbacks on OP::Notifications
Merge remote-tracking branch 'origin/dev' into feature/settings_callbacks
push down callback block check into OpenProject::Notifications
no underscores for private methods
fix a bogus test
Merge pull request #1081 from opf/feature/settings_callbacks