Alternative DeepLab Implementation

[jamesmcclain]

There are alternatives available to the DeepLap implementation in tensorflow/models, for example this one.

At some point in the future, we might find that it is worthwhile to replace our existing backend or create another one to go alongside of it. (The particular concern is whether the inability to get changes upstream into tensorflow/models will cause too much of a problem.)

[jamesmcclain]

See DrSleep/tensorflow-deeplab-resnet#200

[jamesmcclain]

Predecessor of #517