Make world termination thread-safe

When world was terminated twice, the second time the future was not
resolved. This might happen, when calling the termination from more
places (for example in at_exit and trap("SIGINT")).

I've made the call also thread safe, just for sure.
Fix tests

Addressing:

    dynflow/test/test_helper.rb:10:in `block in <top (required)>': uninitialized constant Dynflow::CodeWorkflowExample::PollingService
Use bundle exec to run tests
Unskip test: terminating twice in row doesn't block anymore
Skip test: waiting for remote executor blocks

Known issue, will be addressed soon
Merge pull request #62 from iNecas/thread-safe-terminate

Make world termination idempotent and thread-safe