Add support for config files outside of the source directory

Config files can be specified now with absolute paths with the
DOCKER_REGISTRY_CONFIG variable, for example:

export DOCKER_REGISTRY_CONFIG=/etc/docker-registry.yml

This was initially reported in RH Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1038874
Merge pull request #148 from goldmann/config_location

Add support for config files outside of the source directory