Add size export method.

The method export_sizes() writes down the size of the box, uncompressed
object, and all compressed objects to file in CSV format. This will make
it easier to track all of the compression images that BaxterObjet can
now handle.
Enable dimensions based on min area rectangle.

The min area rectangle is probably the best way to determine the
dimensions of an object, especially if the image xy axes are not aligned
with the object's ideal xy axes.
Add measurement reference object.

The measurement reference object is used to approximate the number of
millimeters per pixel from the camera's vantage point. It does this by
segmenting the reference object and then calculating the conversion
factor based on real dimensions provided by the user.
Make set_compressed_image() handle directories.