Continuous integration fix

This release fixes a path issue that prevented Dockerhub from successfully running the tests.

A few new features

• Can schedule processes with priorities.
• Multiple manager runners

  // Run for a certain amount of time
  Manager& run(high_resolution_clock::duration runtime);
  
  // Run indefinitely or until halt() (TODO: needs C interrupt handler)
  Manager& run();
  
  // Run until a condition becomes true
  Manager& run(std::function<bool()> condition);

  The last method was required to get some tests to pass. For example, the priority methods ran for 100 ms, but on Dockerhub, not much happens in 100ms.
• A new halt() method in processes, which tells the manager to stop, calling all processes' stop methods.
• A flag that should recognize Raspberry pi v3 B in the Makefile.
• A method in State for getting a pointer to the containing state machine.

First version

This is the first stable version of Elma. It includes the process manager, process base class, events, channels, state machines, an HTTP client, examples, and tests.