snapshot testing

Create a snapshot testing system to mirror the super useful snapshot testing capabilities found in Jest (a Javascript testing framework).

Something like assert_that(some_obj).snapshot():
- if no snapshot found, take snapshot and pass test
- if snapshot exists, take new snapshot and compare with existing, pass only if they are equal
Should work for objects, dicts, or any python value (within reason).
Need some kind of deep object verification (see deep dict compare #75)
Snaps should be pickled (or configurable to be json with known loss of fidelity)
Able to ignore a property or branch, maybe ignore=foo.bar.baz
Need unique identifier for every snapshot, maybe filename + line no?

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