Stroop improvements

[davwillev]

This pull request contains a bunch of improvements over the current implementation of the Stroop active task (Objective C):

1. The curent Stroop task does not return a reaction time, only meaningless startTime and endTime captures of systemUptime. I have now added a new result for reactionTime (difference between startTime and endTime)
2. There is currently no result to report that the user correctly matched the button pressed to the first letter of the text displayed at each stage (stimulus) of the task. This could lead to misinterpretation of the result (e.g. a developer could inadvertently evaluate a match between the button and the colour of the text onscreen). I have therefore added a 'match' Boolean result.
3. The current task only returns results for each stage (stimulus) within the entire task (which consists of several stimuli). I have now added some summaries that cumulatively update with each stage over the entire task (mean of reactionTime, standard deviation of reactionTime, percentCorrect of all stimuli so far over the task).
4. The interval time between stimuli is currently fixed at 0.5s. This gives users little flexibiity for changing this between studies or even participants within a study, so it is better that a user can set this interval via a parameter in the task factory. An option for random interval times between two limits within the same Stroop task is also a useful feature. I have therefore implemented all of this using two new parameters - minimumStimulusInterval and maximumStimulusInterval - in line with the reaction time active task. (The method I created to provide random time intervals should be an improvement over the reaction task, which I think contains an error). If minimumStimulusInterval and maximumStimulusInterval are set to the same value (e.g. 0.5) of seconds, then that value will be used throughout the task.
5. I have also moved the spoken instruction to the countdown step as participants found it to be a distraction from the stimuli during testing.