Embeded graph

Hi,

I think I'm the only one that needs this feature, but who knows.
I pull request it, in case someone is interested

What is the idea : Embed Graph in WebUI

I have to publish my webui to internet but because of lack of ACL in graphing tools (easily integrated with webui), I don't want to publish Graphite/PNP. 
People should be able to see graphs.

So, I make shinken ask the graphing tool for the image and then embed it in the html code (as base64 encoding)

By default, embeded graphs are disabled.

Zoom still works. But the "show more" button is redirecting to current page (as the graphing tool is not reachable)

It is already in production for us (using shinken 1.2.4)

What do you think ?
May I enhance some part of the code ?

Feel free to make any suggestion.