Changes to 1.0.15

* New functions CurrentTime, MacaulayRep, MacaulayLowerOperator, MacaulayBound.
* Fix rational powers of negative numbers, and exact negative rational powers.
* Fix zooming in graphs using the mouse when the functions take long to run
* Accept log instead of ln for symbolic derivative when used with only
  one parameter
* Translation updates (Maria Majadas, Juan Matias Olmos, Daniel Mustieles,
  Gustavo Jasso, Jiro Matsuawa, Marek Cernocky, Bruno Brouard, Matej Urbanic,
  Mario Blattermann, Juan Carlos Vallejo Lopez, Christian Kirbach, Jorge
  Gonzales, Martin Strebotnjak, Wolfgang Stoggl, Kjartan Maraas, Fram Dieguez,
  Joe Hansen, Daniel Serbanescu)

* For some of the changes the author (Jiri) was partially supported by
  NSF grant DMS 0900885 and the University of Wisconsin-Madison