Changes

Changes from Mercurial (hg http://isabelle.in.tum.de/repos/isabelle/ default)

Summary

1. added method to generate build schedules directly;
2. clarified load vs. apply vs. make;
3. tuned;
4. tuned heuristic;
5. use cpu time for approximation;
6. lower bound for approximated times;
7. use full timing information in build schedule;
8. consistent hosts ordering;
9. filter hosts properly;