Use PT-Stalk 2.2's --plugin to remove dependency on custom pt-stalk script

[Jericon]

the --plugin feature now adds the ability to define actions to take before and after events (before collect, after sleep, etc).

This would remove the need to use the custom build of pt-stalk, and just make the dependency on percona toolkit 2.2.

http://www.percona.com/doc/percona-toolkit/2.2/pt-stalk.html#cmdoption-pt-stalk--plugin