Move flag assignment to init().
Add 'hosts' command line flag.
Make Pull command work with multiple docker hosts concurrently.

The Pull command can now download an image from S3 and upload it
to multiple docker hosts, called "PullHosts" internally. Downloading and
uploading are both concurrent operations.

PullHosts can be configured using the new `-hosts` command line option.
If `-hosts` is not set, then the PullHost is set to the same as the
DockerHost, and is configured the usual way with config.GetDockerHost().

* The `-hosts` option must be in the following form:

  tcp://host1:port1,tcp://host2:port2

* `dogestry pull` will perform the following steps:

    1. Check each PullHost to see what images need to be
       downloaded from S3.
    2. Download each image concurrently into a temp directory.
    3. Upload a tarball of the temp directory to each PullHost
       concurrently.
Rename command-line option: -hosts to -pullhosts.
Update Readme.md to describe new pull command.

Describe how to use the new `-pullhosts` command line option
to pull an image to multiple docker hosts concurrently.
Add example with -pullhosts option to usage message.
Merge pull request #6 from dogestry/pull-multiple-hosts

Make dogestry pull command work with multiple hosts.