Recognise and enable acceptance of AutoRecorded images.

[babbage]

Complementary feature to the implementation of AutoRecord proposed to iOSSnapshotTestCase in this pull request: https://github.com/uber/ios-snapshot-test-case/pull/32. Enables image to be accepted (“swapped”) as a reference image, when an original reference image does not exist.

[babbage]

This pull request stands alone, but would function best in combination with the following three pull requests: https://github.com/Antondomashnev/FBSnapshotsViewer/pull/69 https://github.com/Antondomashnev/FBSnapshotsViewer/pull/70 https://github.com/Antondomashnev/FBSnapshotsViewer/pull/71

In combination, and with the AutoRecord change to iOSSnapshotTestCase linked above, these changes enable snapshots to be automatically created on first run of a test if a reference image does not exist, to be reviewed, to be accepted or rejected via FBSnapshotsViewer, and in both cases for the test images to be cleaned up afterwards.

[babbage]

Rebased on current master.