Added "Hide On Demand" feature (Progress Indication)

- Added the class method "showProgressInViewController"
- Added the 'show' and 'dismiss' methods for the same
- Replaced "Custom" with "Progress" in the demo for showcase
- Hence added working code sample for progress in the Demo

All the changes in the library have been #pragma marked as "Progress".

TODO:
- Add a Activity Indicator (Spinner) in the Progress Card