support warning and critical environment variables

Like the mainline plugins do.
CPU usage graph, skip list, some cleanup

- xenskip option to skip graph types
  env.xenskip "<space separated module list>"
  Modules: cput, cpup, mem, disk, net
- CPU percentage graph added
Merge pull request #248 from mjholtkamp/master

Added traffic_ipt plugin based on traffic (now with clean fork)
Merge pull request #244 from github-ivan/master

xen-multi CPU usage graph, skip list, some cleanup
Merge pull request #243 from steveschnepp/warncritenv

support warning and critical environment variables