New time display

Hi Thomas,

I've added a new function to the UI of your pympress. It can now optionally display not only the elapsed time, but also the remaining time. The elaped/remaining time display can now be monitored slide-wise and presentation-wise.
Also, I've added the updates from the master branch of ryukenzen's fork of pympress.
I have some further plans to add sound signalling.

Cheers,
Arsen (farwater)