Parser can now parse scalars. Also added new `!channel` tag.

class YamlFactory(yaml.YAMLObject):

    @classmethod

    def from_yaml(cls, loader, node):

        if isinstance(node, yaml.ScalarNode):

            value = loader.construct_scalar(node)

            return cls(value)

        else:

            values = loader.construct_mapping(node, deep=True)

            return cls(**values)

class Channel(YamlFactory):

    yaml_tag = u'!channel'

    def __init__(self, port, insecure_mode, credentials=None):

        self.address = '[::]:{0}'.format(port)

        self.credentials = None

        context = Context()

        if not insecure_mode:

            server_key = credentials['tls-server-key']

            server_cert = credentials['tls-server-cert']

            client_certs = credentials['tls-client-certs']

            self.credentials = context.load_server_credentials(server_key, server_cert, client_certs)

            if not credentials:

                click.echo("ERROR: no TLS keys were specified and no defaults could be found.\n" +

                           "Set `insecure-mode: false` in order to deactivate TLS encryption.\n", err=True)

                sys.exit(-1)

class Disk(YamlFactory):

    yaml_tag = u'!disk-storage'

def get_parser():

    yaml.SafeLoader.add_constructor(Channel.yaml_tag, Channel.from_yaml)

    yaml.SafeLoader.add_constructor(Execution.yaml_tag, Execution.from_yaml)

    yaml.SafeLoader.add_constructor(Action.yaml_tag, Action.from_yaml)

    yaml.SafeLoader.add_constructor(Reference.yaml_tag, Reference.from_yaml)