Multi Level Drop Downs

I enhanced extended-choice-parameter-plugin to support multi level drop downs for hierarchical selections.  I also added some help files.

For example, with a multi level drop down, you could have two drop downs, first to select a country, and second to select a city.  The subsequent drop downs choices depend on the prior selections.

So for example, suppose that there are two country choices: "USA" and "Mexico", and four city choices: "San Francisco", "Chicago", "Cancun", and "Mexico City".  First the user selects "USA", then a select "Select a city" drop drop down becomes visible with the options "San Francisco" and "Chicago" (since these two cities are in the USA).  If the user goes back and instead chooses "Mexico" for the country, then the city drop down choices will now  be "Cancun" and "Mexico" (since these two cities are in Mexico).

I setup a demo at http://173.61.174.142:8080/job/Hierarchical_Dropdown_Demo/build if you'd like to try it out.  (Note that you'll need to login to actually run a build, but you don't need to login to just try out the new drop downs.)  Note that there is no restriction on the number of levels -- for example see this demo with 4 levels: http://173.61.174.142:8080/job/Bam_Plot/build .

Please note that I had a little trouble documenting the "Text Box" and "Radio Buttons" parameter types.  These types don't work in my testing.  I documented them as "experimental".  Please let me know if you'd like me to elaborate on how these two types fail to work.

Please let me know if you have any suggestions for improvement, and I'll submit a new pull request.  Also, please let me know if/how I can be of any assistance in updating the documentation/screenshots at https://wiki.jenkins-ci.org/display/JENKINS/Extended+Choice+Parameter+plugin .