Cache the timestamps of the requests

The timestamps currently isn't cached, such that it may be impossible to determine when a requests was actually made.  This provides an attribute in the API object that tracks the timestamps involved in the previous request that was made.

Ideally this should be integrated into the result set (possibly as some metadata), but I didn't feel like revamping all the classes and the results to do this, so this is a rather quick and dirty fix.

If this is considered too unclean for the overall design (it is a bit dirty) do consider just leaving/rejecting this pull requests and integrate some way to include the timestamps in the cache to be returned alongside with the requested results.