Caching images to make a better 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.
Delete temp file.
Merge pull request #3 from ZikiSoy/master

Caching image files to improve user experience.