png 24 and png 32 support added to addimage

Hi,

i've added png support to addimage. 24 and 32 bit at this point and i'm looking into 8. I've also added the ability to pass ArrayBuffer and ArrayBufferView/TypedArray to addimage, although i'm not sure if this is actually desirable?

I need to get together a list of my additions and the thinking behind them and also an example file. And it also needs a bit of refactoring/optimization. But i just thought i'd send this over now and get the ball rolling.

If there's anything that makes you think 'WTF is this guy on', i'm fairly new to JS, so bare with me...