Implemented Main Configuration File: config.json.

Keys as well as programs are now in a single configuration file as
described in issue #26. key_combinations was renamed to keybindings in
order to remain consistent through our code and configuration.

Signed-off-by: Martin Jorn Rogalla <martin@martinrogalla.com>
Merge pull request #27 from AirWM/configFile

Implemented Main Configuration File: config.json.