issue 77 - bugzilla_components.txt created in wrong folder

https://github.com/mozfr/transvision/issues/77

I've created a "cache" folder (with a .gitignore file to ignore all the files inside it in git) and I've define CACHE to point to this folder in "constants.php" (maybe this is not a good idea?). 

"bugzilla_components.txt" renamed to "bugzilla_components.json" (I think it makes more sense).