Minor refactor to initializing a logger for Reclass

The primary goal here was to drop the need/use of the get_options()
from reclass.config, and to instead rely on a parameter passed in from
the caller (for debug mode). This is meant to hold us over until the
config/option/cli refactor is complete - it will be easy to refactor
logging configuration later.
Move reclass/cli.py to a module, reclass.cli
Define argh-compatible func/cmd and entry points for setuptools

The entry points are used by ``console_scripts``, and are simple
functions leveraging ``argh`` for option/argument parsing and command
dispatching.

This initial commit simply defines the new functions but does not
update Reclass to use them.

Note that the salt/ansible adapter commands are simple place-holders
for the moment.
Update entry points in setup.py to use new argh commands

Also add argh as an installation dependency for the package.
Include config for YamlFS storage in reclass.defaults
Import storage backend module defined through config

Loading a backend storage module (yaml_fs, or others), now expects
the full python path to the module with the ExternalNodeStorage class
to import. This path is defined through the reclass configuration, with
a default to ``reclass.storage.yaml_fs``.

Here is an example, default config:

    inventory_base_uri: /etc/reclass
    storage_type: yaml_fs
    class_mappings: None

    yaml_fs:
      module: 'reclass.storage.yaml_fs'
      nodes_uri: ${inventory_base_uri}/nodes
      classes_uri: ${inventory_base_uri}/manifests
rename reclass.cli.scripts.main() to reclass()