Merge branch 'release/1.1.6' into develop

Release 1.1.6

This release fixes two bugs, including a critical bug that can cause unwanted duplication of tasks (Issue #24).

Fixed:

Closes Issue #23: bug: Adding task with priority enters start date at wrong location.

Closes Issue #24: bug: Reload while filtered on context shows duplicate events.
Added "None of the following" option in the top line of each filter. Addresses Issue #25.
- Added Sparkle Framework to enable automatic updates. Closes Issue #21.
- Organized Preferences window to multi-tab interface. Added preferences related to automatic update checking.
- Bumped version number to 1.2.0.
- Updated About TodoTxtMac text.
- Updated README.md to add FAQs and for other small changes.
Merge branch 'release/1.2.0'