Bay

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- Fix exception when trying to rebuild a volume that was built with an old version of Bay

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- Change registry interface to have separate login, add push command
- Pre-build volume providers when building containers
- Add warnings about old link formats
- Fix towline reading so that it correctly looks for boot status
- Autocomplete working
- Make `volume destroy` fail with a nice error message if instances are using the volume
- `bay mounts` now lists containers that have devmodes, but aren't in the current profile
- Increased DockerAPI timeout to 42s, catching ReadTimeout exception
- Make HttpWait.ready() immediately retry on RemoteDisconnected exception

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- Added auto-publish to pypi in travis configuration

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- Removed force=True on volume destroy which didn't play nicely across API versions.

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- Reversed output of top-level tasks to make pending operations more obvious
- Upgraded Docker binding to 2.0 (to support recent Docker versions)

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- Fixed incompatability with new attrs 17.1 hash default
- Docker-for-Mac detection and caching enabling (selective caching coming in future)
- Allow child profiles to override their parent's default boot config