A macro generating a Creative Commons licence mark.

This is part of addressing #41.
Generate a Creative Commons license mark. Fixes #41.

Declares new configuration variables: CC_LICENSE, CC_LICENSE_DERIVATIVES,
CC_LICENSE_COMMERCIAL, and CC_ATTR_MARKUP. Documentation in README.
Add documentation for generating CC license marks.

This wraps up addressing issue #41.
Update footer documentation to mention license mark.
Fix back-quoting and double-quoting mistakes.
Make licensor overrideable by article author.
Straighten out indentation so we get less confused.
Replace line break with <p> for better text margin.
Merge branch 'cc-license-mark' of https://github.com/hlapp/pelican-bootstrap3 into hlapp-cc-license-mark

* 'cc-license-mark' of https://github.com/hlapp/pelican-bootstrap3:
  Replace line break with <p> for better text margin.
  Straighten out indentation so we get less confused.
  Make licensor overrideable by article author.
  Fix back-quoting and double-quoting mistakes.
  Update footer documentation to mention license mark.
  Add documentation for generating CC license marks.
  Generate a Creative Commons license mark. Fixes #41.
  A macro generating a Creative Commons licence mark.