fixes issue #110 and #128, adding possible_events method

Original commit message:
- removing version fixation on graphviz (issue on my ubuntu 14.04)
- removing quotes to fix #110 (when using spaces, one has to add the right quotes manually)
- adding meta fix by peck fixes #128 (got same problem, thanks for fix)
- added possible_events method to determine possible events by given object context / state