arc helper script to run a container on the Docker host

Summary:
Installation:

wget -O /usr/local/bin/arc https://raw.githubusercontent.com/nasqueron/docker-arcanist/master/contrib/arc
chmod +x /usr/local/bin/arc

Use:

• arc ... to run an arc command
• arc call-conduit uses a kludge to handle stdin/stdout
• arc shell to run a bash shell to run interactive arc commands

Test Plan:

• arc branch
• echo '{}' | arc call-conduit user.whoami
• arc shell

Reviewers: Kaliiixx

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Differential Revision: http://devcentral.nasqueron.org/D145