add multiple images per page

[dduponchel]

These commits add the possibility to display several graphs on a page. If more than one, a tab bar appears to choose the graph.
I also split the code into methods of an object (the code grew too big) and added unit tests.

edit :
as discussed via emails, here is a list of additional features I added :

• all the tabs are shown, only the default tab changes between the pages
• the hidden images are lazy-loaded
• a new "more info" tab which redirect to the kicktraq page

[dduponchel]

I added 3 commits to fix issues on my new code.
On my branch develop-alternate-tabs I also added an extra commit which changes the behavior of the plugin. Instead of displaying 1 image on the project home and 2 images (+ tabs) on the backers page like in the current pull request, we always display the tabs with all the available graphs, the current page only changes the default tab.
If you prefer this behavior, I will add the commit to the pull request.

Edit : the branch develop-alternate-tabs has been merged into the branch development, adding the commits to this pull request. See the updated description.