Add ReportByCpu and ValuesPercentage for cpu plugin

ReportByCpu defaults to true, ValuesPercentage to false

This enables the following use cases:

- ReportByCpu false: emits normalized percentage values
- ValuesPercentage true: emits percentage values
- default: send per core ticks, as previously
handle comments from @octo
fix typos
keep plugin instance for first cpu
add a value_to_rate function.
as advised by @octo move to a rate computing function
typo fixes.
Merge branch 'master' into feature-normalized-cpu
better indentation.
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Merge pull request #516 from pyr/feature-normalized-cpu

Add ReportByCpu and ValuesPercentage for cpu plugin
add documentation for #516
typo in plugin descr header
Merge pull request #575 from amiron/riemann_notification_message

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