Implement diff tool for ASDF files

[drdavella]

This resolves #211 and implements a diff tool that can be used for ASDF files. I don't have a ton of test cases so there may be missing features and/or buggy behavior on certain files.

@nden since this was your request maybe you can take it for a spin when you have a chance and let me know what you think.

[drdavella]

This kind of feature doesn't lend itself very well to unit testing, but it might be good to give some thought to how it can be tested.

[coveralls]

Coverage decreased (-2.5%) to 91.777% when pulling f704aeb on drdavella:asdf-diff into fd5ea50 on spacetelescope:master.

[coveralls]

Coverage decreased (-2.5%) to 91.777% when pulling f704aeb on drdavella:asdf-diff into fd5ea50 on spacetelescope:master.

[nden]

This is really useful when looking at different versions of reference files. 👍 for merging

[drdavella]

Awesome! I think I should probably try to add some kind of tests first though since it drastically reduces overall test coverage...

[coveralls]

Coverage decreased (-0.2%) to 94.153% when pulling 474701e on drdavella:asdf-diff into 44b6a10 on spacetelescope:master.

[coveralls]

Coverage decreased (-0.1%) to 94.194% when pulling 26022af on drdavella:asdf-diff into 44b6a10 on spacetelescope:master.

[coveralls]

Coverage decreased (-0.09%) to 94.216% when pulling b77c8c9 on drdavella:asdf-diff into 44b6a10 on spacetelescope:master.

[coveralls]

Coverage increased (+0.08%) to 94.393% when pulling 0f7df43 on drdavella:asdf-diff into 44b6a10 on spacetelescope:master.