Allow specification of X and Y dimensions for auto resizing (so thumb is no fewer than X pixels in width and Y pixels in height upon resize)

[lthurston]

Often when creating a thumbnail you're not sure the aspect ratio content editors will provide. Rendering a thumb that is no less than X pixels wide or Y pixels tall is useful in these situations. Fortunately, it's also easy to implement, and I've already done it, just needs to be tested on my end. Expect a pull request in the next couple days.

[lthurston]

Pull request #16

[justb81]

maybe one could use the getThumb method from System\Models\File