added simple slabtop summary plugin

## Notes

This is attempt 1 at a scout plugin and I -think- I've got what I need in there.  @gstamp - would you mind taking a look at this, since you were kind enough to point me in the direction of our repo of scout plugins

- can get simple metrics from slabtop
- requires passwordless access to 'sudo slabtop'
- tests successfully on linux using scout test slabtop.rb