modules: add admin stats module.

Work in progress.

Quite usable already, but still needs to add support for multiple line series.
A lot of stats are still not shown.. better overview and configurable update rate
and time span in the graph would be nice, as well as zooming and inspecting
individual samples.

Updated snapshot (the mean values are in microseconds, I just pushed a fix for it...):
![snapshot8](https://f.cloud.github.com/assets/72965/216120/488ba766-84b5-11e2-9cc0-2e195d102991.png)

What you're looking at:

The histogram charts groups requests into bins based on how long time they took to process.
Each bin is represented as a bar in the chart, the height inidicate how many requests went into
that bin, and the position, and width of the bar indicates the range of durations for all requests in the bin.

The line charts show a count value for one minute of activity (not sure how folsom comes up with the value, though).
Just after 11:14, I hit `z:flush()` as is pretty clear from the db requests graph, and at 11:15:30 I hit reload
on a page repeatedly, which reveals that each request hits the db, but the impact isn't as bad as a empty cache ;)

And, as can be seen, as soon as the activity stops, the line charts starts falling.. as they follow the one minute
category. There's categories for five, fifteen and day as well, not yet shown, but that wouldn't be as easy to budge
using a single page session.

--
Old snapshot using the nvd3 lib, no longer applies...
![snapshot6](https://f.cloud.github.com/assets/72965/191753/a42e1e3c-7f47-11e2-811d-6261c95c02cb.png)