added global instance to cpu plugin to get average metrics on Linux

I think this approach is much better in very big servers ( I usually work in servers with up to 40 cores ) and is easier to parse a single line on /proc/stats than calculate 40* 8 ( metrics/core) averages values. 

It also makes to have simpler config files ( avoiding average plugin)

SOME DETAILED INFO

This metric is calculated by parse /proc/stat first line ( which countains the aggregation of all the other cpuX lines.

http://www.linuxhowtos.org/System/procstat.htm

And after I divide by the number of cores taken from /proc/cpuinfo in the init section of the plugin.