Add -O switch, which directs cpudist to collect off-CPU time
statistics. Also restructure the code slightly and added examples
as appropriate.

* cpudist: summarize on-CPU time per task as a histogram

This is the initial implementation of the tool itself, which uses
the sched:sched_switch tracepoint to probe task switches. This is
a slightly more robust approach than using a kernel function, such
as finish_task_switch.

When native BCC support for tracepoints is introduced, this tool
can be reimplemented and reliance on the current Tracepoint module
can be removed.

* cpudist: add man page and examples