Added the generation of an index page

Hi,

First I want to thank you for this Jenkins/Concordion plugin.
I saw that the index page generation was still in the TODO list, and I needed this feature too, so I thought I could just code it :)

The generated page is very basic, with some minimalist embedded CSS styles, and I added an option to reduce the names of the links to the actual reports, to enhance readbility.

Hope it helps,
Olivier