Simple image box arithmetic

## Project description

This provides image crop/resize algorithm for chaining multiple resize, crop actions and producing a resulting crop/resize action pair.

## Usage

The usage is fairly simple:

```from boxmath import box, resize, crop, size, make_transformer
from wand import image

# Load the image to get its width and height
i = image.Image(filename="chrysanthemum.jpg")
b = box(i.width, i.height)

# manipulate the virtual image
b = resize(b, 629, 483)
b = crop(b, 0, 0, 480, 480)
b = resize(b, 1000, 1000)

# render
def resizer(img, w, h):
img.resize(int(w), int(h), filter=FILTER)
return img

def cropper(img, l,t,r,b):
img.crop(int(l),int(t),int(r),int(b))
return img

t = make_transformer(b, resizer, cropper)
i = t(i)
i.save(filename="chrysanthemum-1000x1000.jpg")
```

Normally, if we would of used wand or PIL directly, each resize would degrade the image. The action of down scaling and then up scaling would wreck the quality of the image; with the power of math, we only apply the resize and crop when we need render the image.

Not that the width, height, left, top, right, and bottom values passed to the resizer and cropper functions are cast as ints.

This is because they are either fractions.Fraction() instances or int(). boxmath uses the Fraction class to ensure precision while resizing and cropping.