I never realized that "it's" could mean "it is" and "it has".. I get it though.. I just can't recall ever seeing "it has" spelled that way..
although I don't remember english class that well(I learned most of my english from tv and internet/games, not at school, it was one of those classes where I never had to pay attention), but I seem to remember being told in class that 's could mean is or has. at first we always had to write it out, only later on were we allowed to use contractions.
funny though that those contractions are so common in english, in dutch we only have the possesive 's, and, rarely used, 't for het(it)(wich is usually used on it's own, not contracted). in frisian we do contract the you-form (you have=do hast=hasto), but none of the other forms