New plugin: turbostat (CPU frequency and C-state residency)

This plugin is based on https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/tools/power/x86/turbostat/turbostat.c

It reads CPU frequency and C-state residency on modern Intel turbo-capable processors.

Here are examples of produced data (using CGP, see https://github.com/Feandil/collectd-turbostat/tree/master/CGP-plugin for the CGP plugin).
Example 1 (Small load):
![collectd-turbostat_cgp](https://cloud.githubusercontent.com/assets/1488247/3334380/fb54fb0a-f7fe-11e3-9f4c-67b501030705.png)
Example 2 (Big compilation near the end):
![collectd-turbostat_cgp-2](https://cloud.githubusercontent.com/assets/1488247/3334381/039a95d6-f7ff-11e3-818d-3684ae7cabc1.png)





I've just tried removing it (a76020562109f497b8076fd9d562edfff5961073), but it results in the following warnings:
```
Aug 21 23:20:38 collectd[10604]:  Found a configuration for the `turbostat' plugin, but the plugin isn't loaded or didn't register a configuration callback.
Aug 21 23:20:38 collectd[10607]:  Found a configuration for the `turbostat' plugin, but the plugin isn't loaded or didn't register a configuration callback.
```