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)





Well, I guess it's a reflex, just to be sure that I know why it failed.
In most cases, there is an "ERROR" message just before, so we don't really care.
I'll check the other cases, add the error messages and replace the error codes with -1.