Add alias methods for better reading of code

[program-the-brain-not-the-heartbeat]

Right now I have to:

$rrule->getRule()['INTERVAL']
$rrule->getRule()['FREQ']

Would look cleaner if it was $rrule->getFrequency() or $rrule->getInterval() which could just return $this->getRule()['FREQ'] etc.

[rlanvin]

That sounds like a good idea. Will you submit a PR?