Add parameter to reduce min num samples for stats

- min-samples parameter (default 30) is the minimum
  number of trials (samples) for calculating statistics
- benchmark/examples/debug.rkt is an example
- Fixes #14
Add racket wrapper for mk-shell-benchmark

- mk-racket-file-bench takes name, file name, list of arguments to
  pass to racket
- mk-racket-file-bench runs 'raco make <file name>' as its build step
  by default
- rename mk-shell-benchmark to mk-shell-bench to be consistent
  with other names
- update benchmark/examples/jit-no-jit.rkt to make use of
  mk-racket-file-bench
- fixes issue #12