travis-ci: start petri service
travis-ci:  don't block on `git show`
travis-ci: use absolute path for logdir
travis-ci: try readlink -f instead of realpath
traivs-ci: install the petri module

We need to do this (or similar) so that the devserver script can find
the `ptero` executable.
system test framework 'runs'
remove non-model operation type builders
single_operation test uses `pass-through` op type
get database string from environment
set database string in system tests
send net to petri and listen for operation events
get net start url more cleanly
simple workflow has flow control, but no outputs
pass color_group to backend.event()
make impl/modules a sub package
remove stale test database files
devserver sets up basic logging config
extract model classes into separate modules
extract basic operation subclasses
extract Operation.subclass_for