Add option to monitor single pipelines and to monitor events in/out per minute.

[TobiasBielefeld]

Hi,
we wanted to have extended features for this plugin for our use case, which includes:

• Possibility to monitor only one single pipeline
• Possibility to monitor the incoming/outgoing events on a per minute base.

We especially wanted the events per minute metrics, since our inflight events were zero very often. The new metrics allow us to see that events are processed by checking if the events in/out per minute is above zero.

Other smaller adjustments

• Thresholds can now be correctly given where only min: is used
• Refactoring to have less code

I didn't touch the plugin in the root directly. I thought that it will get automatically replaced via rake.

[MarcusCaepio]

@widhalmt We really would like to get this merged, so we can create a PR in the Icinga Repo for the ITL. This also fixes #45

[martialblog]

We added the option to select a pipeline name in the new release.

We will evaluate if the per minute events will be included in a later release

See #47