Initial implementation of drawing graphs

I've added the initial implementation of drawing graphs.
I use the jQuery plugin "Flot" (MIT license) to draw graphs:

* http://www.flotcharts.org/

Currently there is no way to choose the time range, it shows just only recent 6 hours.
But it can zoom a part of the time range by dagging the graph area.

![graph](https://cloud.githubusercontent.com/assets/135104/5117710/8ab1b300-709b-11e4-84c1-604b6fdeeff7.png)