Added method resizeToBestFit

[Droeftoeter]

Added a method to resize an image to best fit a given set of dimensions
while keeping the original aspect ratio.

[ghost]

I think this should resolve what the OP of #24 was trying to achieve, although without chaining.

[ghost]

Should it return $this->resize($width, $height, true) or $this->resize($width, $height, $allow_enlarge) or does it not matter?

[Droeftoeter]

It doesn't matter, aslong as it's either true or passes the $allow_enlarge on. It already does nothing when both dimensions are smaller than the provided box and allow_enlarge is false.