Add an option for roslaunch, and add a sleep parameter

[xrkong]

Function update:

1. Add a launch file named launch/publisher.launch
2. Add a parameter named sleep

Function 1 is for another way to launch a ros node.
Function 2 is for the requirements that users want to start other nodes before this node. Because I use this publisher to test my image process codes, I want to start some nodes before the image publisher.
Using sleep parameter I can easily initialize other time-cost process nodes and process every images in the folder.