[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
DDF-458: Updated to build on JDK7 and JDK8.
DDF-458: Commented out JDK8.
Added DDF JIRA ticket URL.
[DDF-476] Fix API documentation errors so that newer javadoc (JDK8) will build

Also fix some warnings and other docu issues noted during work.
Merge pull request #7 from bradh/DDF-476

[DDF-476] Fix API documentation errors so that newer javadoc (JDK8) builds
Reviewed by: @mrmateo