Histroy data view

Now, a collection of host statics are displayed to the user in a real time fashion. The user can know what is going on by monitoring the host statics. However, sometimes, the user need to look back into the host statics for system management analysis, especially he need to get the statics in a certain time range.

In the front end, a coordinate diagram within a certain time should be displayed based on the statics data from the backend. And the front end may or may not cache the statics data. In the back end, a file should be created to store the host statics sampled in a certain rate, say 100 seconds by default. The API for the client to get the samples will be

GET /host/stats/io {"time-span": "2013-11-11:00:00 - 2013-11-12:00:00"} return {"samples": "rate1 rate2 rate3 ..."}