added sha1 support for create-rfm-from-url-list

[ccacciari]

I wanted to use the utitlity create-rfm-from-url-list to generate a BagIt package using a OwnCloud webdav url. However, as explained here: https://central.owncloud.org/t/reading-hashes-md5-or-sha1-via-webdav/14348, OwnCloud uses sha1, so I added the support for sha1. I also added some logic to remove the sha1 prefix and to process the checksum when it is not base64 encoded.

[coveralls]

Coverage remained the same at 96.355% when pulling d2c02ff on ccacciari:master into 9b2b728 on fair-research:master.

[mikedarcy]

This PR looks fine to me. I'll include it in the upcoming 1.6.2 release. I'll probably fiddle with head_for_headers a little and allow a config to be passed to it rather than just having it always default to DEFAULT_CONFIG. I also need to add some notes about bdbag_utils.py and that it is essentially "experimental" since none of this code has unit tests. Thanks for the contribution.