Add filter function unit tests

[danheberden]

Need to test things like

gith({
  repo: function( payload ) {
     return /gith/.test( payload.repo );
  }
}).on( 'all', function( payload ) {
  /* do stuff */
});

and create/test

gith( function( payload ) {
  /* check payload for stuff */
  this.emit( 'customevent', payload );
}).on( 'customevent', function( payload ) {
  /* yay! */
});