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.
Update cross and tgz to play nicely together (creating a tgz for each supported OS/ARCH), and update release.sh to upload binaries and tgz files for all the supported OS/ARCH combos
Fixed #3039 - Added clarify on port options in API
Merge pull request #3340 from jamtur01/apiport

Fixed #3039 - Added clarification on port options in API
Merge pull request #3337 from tianon/release-darwin

Release tgz and binaries for cross-compiled platforms
Merge pull request #2809 from graydon/880-cache-ADD-commands-in-dockerfiles

Issue #880 - cache ADD commands in dockerfiles