Screen recording for each session

This is a feature that can record test scenario into sliders. The test scenario includes "click", "send Keys" and "save screen shot" actions to browser. The selenium grid can save screen shot when any of those action is performed, and high light the web element that is involved, eg: the button being pressed, in the screen shot.

To enable the feature, adding "-screenSliders \<path to store slider files\>" option in hub, eg:
java -jar build/java/server/src/org/openqa/grid/selenium/selenium-standalone.jar -role hub -screenSliders /tmp

To view recorded screen shot sliders, open url in browser:
http://\<hub addr\>/FilesServlet?sliders=\<session id\>

eg:
http://127.0.0.1:4444/FilesServlet?sliders=772e55a0-fa59-496c-996e-73b4ed34d0e5


The change sets includes:
1. new java servlet to create and view sliders
2. an images viewer in html page
3. grid code that can 
  A. create screen shot when "click", "send Keys" and "save screen shot" actions are performed
  B. load configuration
4. unit test for new codes