Adding support for "usage" and "creation" search endpoints

The following documented endpoints are missing from the gem.

http://www.jfrog.com/confluence/display/RTF/Artifactory+REST+API#ArtifactoryRESTAPI-ArtifactsNotDownloadedSince
http://www.jfrog.com/confluence/display/RTF/Artifactory+REST+API#ArtifactoryRESTAPI-ArtifactsCreatedinDateRange

This doesn't get the gem fully up to spec with the documentation. I'll admit that I'm only adding what I intend to use... To that end, I _have_ installed this locally to further verify functionality.

Good gem. Thanks for creating it! :beers: