#12 Single Instance

[AndreLDM]

Implement a single instance mechanism that works fine on Windows and Linux.

Useful reference:
http://stackoverflow.com/questions/171213/how-to-block-running-two-instances-of-the-same-program
http://rosettacode.org/wiki/Determine_if_only_one_instance_is_running#C

[AndreLDM]

This feature should enable the player to open multiple files at once in the same instance. If the user opens another group of files, it should replace the ones already open.

[AndreLDM]

Moved to GitHub:
https://github.com/andreldm/kissplayer/issues/8