Hi, I have a pitbull mix too (pitbull/american bulldog) and went through the same thing.  I just bought an abundance of toys, and made sure to have one handy all the time.  Whenever Kweli (my pit) would start to bite, I would sternly tell him NO, and then replace my hand, wrist, finger, or whatever with the toy.  It did wonders (although I know it's simple and might not work for you) and eventually he learned to play without biting.  After than, we worked on allowing him to playfully naw VERY gently on hands/fingers. 

the toy thing worked for me too...it is annoying to have to go grab one every time he started in but it was better than getting nipped at all the time!

puppies theeth till good 16 weeks , a nippin puppy can pose a problem so the best thing to do is repremand them  by a sharp but not yelling no or bad always use the same word so your puppy can understand mulitply commands do confuse .