Add a plugin for monitoring zookeeper

This adds a simple plugin to read data from Zookeeper's MNTR
command to allow for easy monitoring of Zookeeper from collectd
without requiring jmx
Add information to collectd.conf (5) for the zookeeper plugin
Use generic types for a few more of the zookeeper metrics
cpu plugin: Fix ValuesPercentage to behave as documented.

The documentation claims that ValuesPercentage is only considered when
!ByState && !ByCpu. Fix the behavior to match this documented behavior.
This makes cpu_commit_without_aggregation much easier.

Bug: #820
cpu plugin: Mark the "active" state as available.

This fixes the !ByState && ByCpu case, i.e. when the user requests one
metric ("active") per CPU.

Bug: #820
Merge pull request #826 from katzj/zookeeper

Add a plugin for monitoring zookeeper
reintroduce counter, killed by error
reset counter type to it's initial value

It was a COUNTER, not a GAUGE before it got accidentally deleted by
6372ca6a38.
zookeeper: add mention in README & AUTHORS
RPM specfile: add zookeeper plugin