added centerOn, changed maxZoom method

[theJenix]

Added a centerOn method, to programmatically center on a point in the image.

Changed the maxZoom method to better support rectangular images

[theJenix]

Sorry this is in 2 commits...I had other ideas with the first commit and was under a deadline. The second commit reverts a lot of superfluous changes, and gets just the core functionality I want to push back into the main line.

[SalmanTKhan]

theJenix, very nice contribution, is there a way to keep the zoomed image rectangle bounds within image's bounds rather than the outside square, what I wanted to do was actually use this in conjunction with "option to force fill image to view" but it didn't work out so I tried zooming before and then using your center code.

[weakapp]

@SalmanTKhan : you can use scrollBy instead of using postTranslate, that way you may keep you image bounded.