Munin Plugin Support

![scout 390da7284bd9](https://cloud.githubusercontent.com/assets/7880/4583998/db963e5a-4ff6-11e4-9a81-d1d9b7e78a6c.png)

Folks frequently switch from other monitoring systems to Scout. It'd be great to be able to run plugins from other systems to ease the migration. 
This is the start of adding that support. The agent will automatically run munin plugins found in `/etc/munin/plugins` via `munin-run`. 

Thoughts:

## Flag to run Munin Plugins

I don't want to run Munin plugins by default (seems like unexpected behavior). I'm thinking through the following possibilities:

* `scout KEY -munin` (looks in `/etc/munin/plugins` and tries to run those plugins)
* `scout KEY -munin PATH` (run plugins in the specified `PATH`)

The second option would allow users to run plugins selectivly (copy the munin plugins they want to run via Scout to a specific directory).

## Realtime

These are treated as local plugins and realtime isn't supported with local plugins. 

## Roles

Similar to realtime, these are treated like local plugins so triggers can't be added via roles.

## Duplication

Some default munin plugins are very similar to Scout's autodetect metrics. Munin typically reports more metrics though. These are potential overlapping defaults:

* cpu
* diskstats
* if_eth0
* load
* memory (but folks have mentioned they like some of the munin memory metrics)

## Next

First, some general cleanup and error handling.

To get started, I'll reach out to customers that indicate they are transitioning from munin and let them know about this. Once we know this works, we can add public-facing documentation.