Add support for building contexts objects

[moysesb]

This PR implements #538 and is necessary for docker-compose PR docker/compose#1244

[aanand]

This should probably just say "Docker" 😏

[aanand]

1. How much of this logic is actually implemented in the Docker client? For example, validation of the Dockerfile seems to happen on the server side. We don't need to go beyond what the Docker client implements.
2. Documentation would be great.

[moysesb]

@aanand

1. Scaled back the validation to be just on access rights, Dockerfile name check and tarball format. The docker go client does roughly that level of validation; it supports reading a compressed context from stdin and should implicitly reject a malformed tarball (not sure about whether it actually rejects it, but I'd consider it the right thing to do).
2. Added documentation at docs/context.md

[aanand]

Thanks. Looking good.