added unit test for (un)tagging functionality, closes #131.
also edited the tagging method in Task to store all tags in lowercase.
(debugged) does not check if task is already done when trying to mark done in show tag view.
Modified the InvalidIndex function to check for max index of current list displayed rather than the max index of the undone tasks
modified getDateRangeTask method for repeat tasks:
- (debugged) was not showing the correct tasks
- (refactoring) the check for the times/days were extracted out to new methods