Add monitor to check for the status of a raid array

This monitor will read '/proc/mdstat' and parse it for several values.

[n/m] where n is the total number of devices and m is the number of working devics.

[UUU_UU] where U means the specific component is up and _ means that it is down
It will also check for a recovery status in the case of a failed component

The alerting breaks down as follows

[green]:
  n == m
  no recovery state
[yellow]
  n > m
  recovery state
[red]
  n > m
  no recovery state
  array is !active

When defining active using mdstats, active implies that the device is up.  It can be in one of
several states such as sync, clean, etc
Fix portability issue and add docs

I added a documentation header to the monitor with links as to where further info can be found

I added a sanity check to confirm that '/proc/mdstat' was in fact present before trying to open it.
If it is not present then the script will exit gracefully saying the file is not present.
If the file is present but raid is not being used then the script will exit gracefully as the regex
  will return nothing