Configuration options update

[jbrown-xentity]

This adds a configuration option to not add the CKAN API authorization when using an external storage plugin. Tested in GSA's environment, and all worked as expected. Added documentation for this configuration, as well as some light template for other configuration options.

To fix issue #116

[mbocevski]

@jbrown-xentity which external storage plugin are you using? Cause this should be fixed within that plugin rather then what you are suggesting with this PR. Cause this will only work for public S3 objects. One way to do handle this in the external storage plugin would be to generate presigned URL's for the uploaded resource in S3, like for example https://github.com/keitaroinc/ckanext-s3filestore/blob/master/ckanext/s3filestore/controller.py#L72-L78

[amercader]

@jbrown-xentity is this issue happening with the latest version of DataPusher? This seems to suggest it might had been fixed when refactoring to use requests. If it's still an issue with the external storage plugin you are using, I'd be happy to merge this but with a more specific name for the config option (eg INCLUDE_AUTH_HEADER (default `True.)
Also note that when #207 gets merged I added a proper config section in the docs, so the behaviour of the config should be described there.