Delta

Added Counter#delta= for incrementing the value of counters as pulling the last value of a counter, incrementing the value in ruby and then sending the value back to ducks board will result in race conditions. Delta is in the docs here: http://dev.ducksboard.com/apidoc/push-api/

Also, this probably should be in a different pull request, but I added guard to run the tests automatically when files are saved, and restructured the test directory slightly so it is the same as the actual code. Suggestion: As you are using minitest spec in your assertions, it would probably make sense to move all of the test stuff into a spec folder instead.