Add centos/redhat 7 support

The version scheme between RHEL7 and CentOS7 differ. This enburdens
us to make quarterly changes to keep this cookbook functioning if
we are explicit with versions.
By making systemd 'default' for rhel family, we can take advantage
of the regular version scheme of 6.x to correctly identify them
as 'sysv'. So, 'systemd' is now the default for this family.
Add el6.6 to attributes
Added docker_container action :create (wrapper to 'docker create').
Made container_action :redeploy use create or run depending on running?
Made docker_container action :remove remove the actual upstart service file.
Merge pull request #244 from jperville/fix-container-action-remove-with-upstart-service

Made docker_container action :remove remove the actual upstart service file
Merge pull request #242 from jperville/container-action-create

Added docker_container action :create (wrapper to 'docker create').
Added Gitter badge
Merge pull request #245 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md
Merge pull request #208 from intoximeters/el7

Add centos/redhat 7 support