test suite: Added tests for Options.getOptions
Refactoring in Options

Options are specified as functions that modify the default configuration.
This also causes slightly different behavior in the command line
interface: --sender can be specified multiple times without raising an
error now, the last given sender will be used.
Specify receivers not as arguments but as options (with --receiver).
Honor order of command line arguments.
Implement --config option for reading config files.
Allow to specify receivers for specific message sources.

Fixes #6.
less confusing imports