Added index property to Recurrence

[robinvda]

Added an index property to the Recurrence class, which specifies the index of the occurrence.

Get/set the index using the getter/setter on a Recurrence object: getIndex() and setIndex($int).

Note that passing false as the third parameter of ArrayTransformer's transform method will make the index start at the start of the given constraint.

I noticed this is mentioned in issue 108.

[simshaun]

Thanks!