Caching image files to improve user experience.

Using localStorage and backgroud.html to caching images and JSONs.
1. Caching the JSONs in localStorage so that it will not query the
server every time you open a new tab.
2. Every image show next time will be opened in background.html so that
when open a new tab, the image can be loaded from cache.
3. When refresh a new tab (not open a new tab) , chrome still need to
contact the server(but response a "304 not modified"), can't be solved
by now.