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 -30mins` should ensure that we are in an error state for at
least 30 mins.

I've set the defaults to be '> 0' for warning, '> 10' 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.
Adjust swap defaults

Now we have swap-out monitoring, we can ease off the swap-free thresholds
Update CHANGELOG
Add 'playbook' support to our checks.

A playbook is a link to a documentation source describing the check and
potential resolutions. It should be written with a view to an
inexperienced member of staff being able to understand the issue.
Fix error in playbook handling; update CHANGELOG
Merge pull request #32 from ministryofjustice/graphite_swap_out

Add a swap-out graphite-data check