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)





Please document that `t2`, `c2` and `p2` must hold the previous counter values when calling and will contain the delta when returning. Alternatively (and IMHO slightly preferable) you could add a third set of "delta_thread" or "ret_thread" arguments and mark both, `t` and `t2` as `const`.