WIP: Add a swap-out graphite-data check

Checking for actual paging is a better check than monitoring the
amount of swap used -- primarily in that it will alert when paging
occurs.

When the paging stops the alert will clear, so it's important to
check this over a reasonable period of time - in this instance the
`--from -15mins` should ensure that we are in an error state for at
least 15 mins.

I've set the defaults to be '> 0' for warning, '> 50' for critical,
mostly because we will want to be aware of any paging activity. We
may want to adjust this to be less sensitive later.

As the 'swap-used' check is now not required for checking that we are paging, i've adjusted the thresholds so that it will be on less of a hair trigger: `-w 80 -c 95` -- this effectively makes it a check for approaching running out of swap space (and hence warns us that we could see Mr OOM Killer).

Yep, for now it will raise a warning on *any* paging in the last 15 minutes.

I'm of the opinion that we should never page out. Technically, it's not a problem if it is for certain operations at certain times on certain servers, but I think it's a sensible default stance.