Angelic Benediction: Adhere to the 'may' in the card text.

The card text says "you may tap target creature", but the implementation
would always result in tapping the  target creature. This can make a
difference if only Angelic Benediction and Guardian Lions are on the
battlefield, Guardian Lions attacks, yet you want to keep it untapped 
(eg. for blocking potential opponent's creatures with Haste).
Merge pull request #50 from gseitz/angelic_benediction_fix

Angelic Benediction: Adhere to the 'may' in the card text.