Add a 3ware RAID check

Adding a 3ware raid check. This change adds the code for the check
itself, modifies sensu_check to accept "command" and "occurrences" as
well as adds sudoers permissions for the tw-cli command.
Move conditional RAID logic to own plugin

Instead of having the logic in the check defintion itself, move it to
its own check and that will call out to the supported RAID type checks.
Merge pull request #343 from craigtracey/3warecheck

Add a 3ware RAID check