Updated version for next release cycle
Show the tag in 'Built image' output, if supplied.
Use DOCKER_HOST to specify endpoint

For consistency with boot2docker, use the DOCKER_HOST environment
variable to specify the endpoint.  DOCKER_ENDPOINT will still override
DOCKER_HOST.
Use DOCKER_CERT_PATH to find client cert
Remove core.async dependency

Fixes #21
Add support for https and auth identity

Allows use of DOCKER_CERT_PATH for TLS credentials to use for
DOCKER_HOST, as used by boot2docker.

This also allows for DOCKER_AUTH=identity, as used by nodes started by
docker machine.
Updated project.clj, release notes and readme for 0.4.0
Merge release/0.4.0