Glance API ListImageOptions should support changesSince()

The work necessary to fix [`PaginationOptions`](https://issues.apache.org/jira/browse/JCLOUDS-686) in the main jclouds repo requires this downstream change to happen *prior* so the build doesn't break. :+1: 
See related comment on the other PR...a variable named `ifModifiedSince` sounds more like a boolean to me? The value he somehow represents the "search threshold", no? So "modifiedAfter" or "modifiedSince" or "modifiedSinceDate" or so?