Admiring Murdock

• Supporting v2 compose : parsing and handling networks and volumes.
• Completing pull and down commands.
• Add the events command.
• Implementing depend_on directive.
• Add support for build-args in compose files.
• Using context.Context across the API to ease integration with other piece.
• Project refactoring using options struct (explicit) instead of Context fields (implicit). Also adding a APIProject interface.
• Builder split into its own package.
• Switch from candieyaml to go-yaml as it's more active and now has a compatible license.
• A few other refactoring for quality, extensibility and simplification.

Thanks @joshwget @mikedougherty @ibuildthecloud @kunalkushwaha @janeczku @dansteen @bfosberry @dtan4