Factor cache-probing logic out of buildfile.commit() and CmdRun().
Record added-tree sha256 in buildfile.CmdAdd, probe and use cache.
Add testcases for ADD caching, closes #880.
Add self to AUTHORS.
Add CHANGELOG.md entry for ADD caching.
Merge pull request #2809 from graydon/880-cache-ADD-commands-in-dockerfiles

Issue #880 - cache ADD commands in dockerfiles