This change implements communication with an external credentials store,
ala git-credential-helper. The client falls back the plain text store,
what we're currently using, if there is no remote store configured.
It shells out to helper program when a credential store is
configured. Those programs can be implemented with any language as long as they
follow the convention to pass arguments and information.
There is an implementation for the OS X keychain in https://github.com/calavera/docker-credential-helpers.
That package also provides basic structure to create other helpers.
Signed-off-by: David Calavera <david.calavera@gmail.com>
