Adds support for custom pt-stalk triggers

This allows us to provide multiple test cases in which pt-stalk should collect server data
To use original pt-stalk functionality, leave PT_STALK_FUNCTION in scripts/raingauge_rc empty
Add/remove custom triggers in raingauge_triggers.sh
Refer to /var/log/pt-stalk.log to see which trigger caused collection
Adds function get_delta in raingauge_triggers.sh to calculate change in counter values
Merge remote-tracking branch 'upstream/master' into custom_triggers2
Address comments on pull request #21
Merge pull request #21 from mattagra/custom_triggers2

Adds support for custom pt-stalk triggers

This change adds support to let raingauge take advantage of a pt-stalk feature
to create a custom trigger function 
(Details found here: http://www.percona.com/doc/percona-toolkit/2.2/pt-stalk.html#cmdoption-pt-stalk--function).

Instead of defining only one variable and a threshold, we can point to a script which can run multiple
tests and trigger a collection on any of them.  This allows us to monitor threads and cpu load
and connections on the same server for example.  The script is flexible and lets the user customize.