improve protability of config location

[cod3monk]

config.ini location should not be hardcoded to be in the module directory.

I propose using /etc/fablabkasse/config.ini and ~/.fablabkasse/config.ini and allow adding another location via a command line argument.

This would also help with #47.

[patkan]

This package might help with determining the right location to search for a config: https://pypi.python.org/pypi/appdirs/1.2.0