Add scripts for automated performance testing

Add scripts for automated performance testing

This patch introduces two new scripts inside tests/perf: perf.sh and
ag.sh. They depend on being used in a git repository.

perf.sh can run the C tests with different parameters and graph the
results. The results will be named after the current git commit. A single
run takes about 9 minutes on an Intel Core 2 Duo E8400 (3.0Ghz).

ag.sh can aggregate the results of the perf.sh runs across different
commits, creating comparative graphs.

Together, they can be used to track performance regressions in the common
operations.

Signed-off-by: Alberto Bertogli <albertito@blitiri.com.ar>