added simple layout (textview + button)
added CountDownTimer
added button to start entire exercise
Extracted counter logic from activity
Minor changes to UI. Default timer duration set to 90 sec
Brief implementation of workflow: Exercise -> Rest (timer) -> Exercise -> Rest(timer) etc.
added format %02d:%02d to timer textview
Merge pull request #6 from MyDogTom/ExerciseCounter-timer

Exercise counter timer