Make bench.out in json format

This patch changes the output format of the main benchmark output file
(bench.out) to an extensible format.  I chose JSON over XML because in
addition to being extensible, it is also not too verbose.
Additionally it has good support in python.

The significant change I have made in terms of functionality is to put
timing information as an attribute in JSON instead of a string and to
do that, there is a separate program that prints out a JSON snippet
mentioning the type of timing (hp_timing or clock_gettime).  The mean
timing has now changed from iterations per unit to actual timing per
iteration.