[DDF-456] Switch to slf4j in ddf-content
[DDF-456] Remove a left-over bit of log4j
[DDF-456] Fix missing parameter markers.

Thanks to Matt Ramey for pointing this out during review.
Merge pull request #6 from bradh/DDF-456

[DDF-456] Switch to slf4j in ddf-content
Reviewed by: @mrmateo & @shaundmorris