add megaraid.sh plugin to monitor drives behind DELL PERC/LSI megaraid controllers

This plugins allows for monitoring of drives behind DELL PERC/LSI megaraid controllers.
Turns out that recent versions of smartmontools can discover drives and query them for their smart counters and status through a bunch of controllers.

Both prefail and failed conditions should be detected (as reported by smartctl) but I have yet to find a server with drives in prefail state (I've seen a bunch of failed drives tho).

Here's the result with one failed drive:
![megaraid_email](https://cloud.githubusercontent.com/assets/90988/5249936/c8f7f0f8-7987-11e4-9b16-fbfb18e15559.PNG)