New feature: couple countdown with list of speakers

'Begin speach' button starts countdown, 'end speach' button stops countdown.
Clean up main.py and move code to __main__.py and utils/main.py.
fix gui/portable for pull request #903
Some changes in __main__.py and utils.main and other.

Reinsert --no-browser option.

Move some code to utils.main.

Some other small style changes.

Fix merge conflict misstakes.

Change settings behavior. Add tests.

Fix rebase problems.
Changed begin speach behavior. Fix merge conflicts. Add tests.
Merge pull request #903 from normanjaeckel/CleanUpMain

Clean up main and move the code to __main__.py
Merge pull request #920 from normanjaeckel/CountdownAndAgenda

Countdown and agenda
Melt INSTALL.txt, THANKS and README.txt to new README.rst