Convert BaxterObject's compress_obj to list.

Previously, the compressed object could only be one image. Now, multiple
compressed object images can be set in a single BaxterObject object.
Export and dimension methods have an option to either output results for
all the images, or just for the one with the minimum area object
dimensions.
Merge remote-tracking branch 'origin/master' into mult-compression
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.
Add example 8 test case images.
Add example 8 test script.
Add measure reference methods.

export_sizes() now writes information about the reference measure object
to file. A method get_mesaure_size() for grabbing the measure object's
pixel dimensions was also added.
Make set_compressed_image() handle directories.
Merge remote-tracking branch 'origin/measure-calibration' into mult-compression
Add export of spring constant.

The spring constant is calculated for the convenience of the user if
force values for compression images were provided.
Merge pull request #5 from vancezuo/mult-compression

Mult compression