Remove code duplication by introducing a utils library.

Until now collectors didn't really have a good way of sharing code.
We now setup PYTHONPATH so that Python modules under collectors/lib
and collectors/etc can easily be imported by individual collectors.
The most common code duplication was around dropping privileges, so
start by cleaning that up.
procstats.py: Collect interrupt statistics from /proc/interrupts

Collect per CPU interrupts statistics from /proc/interrupts.
Metric name: proc.interrupts
Interrupt types are classified per "type" tag.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>