Better docker and nested workflow support.

This release introduces three runtime options

docker_file
docker_image
docker_volumes

Option docker_file import a docker file, docker_image pull or use an existing docker image, and docker_volumes specified how to share local disks to the docker container. With docker_image specified, all script-executing actions such as run and python would execute script inside the container.

This release also significantly changes the implementation of nested workflows. Its syntax has been changed from

[A=B+C: source='bc.sos']

to

[A]
sos_run('B+C', source='bc.sos')

for a cleaner syntax and more flexibility. Please refer to the SoS documentation for details.