Add benchmark script.
Refactor logger.

- When no logger is set, avoid the computational
  work of `log`, `request_summary` and `response_summary`.
- This can have a noticable impact for suites that
  do lots of VCR logging (see #311).
- This is an alternate form of the improvement
  provided in #311, with a couple further improvements:
  - Retain benefits of null object pattern, rather
    than littering these methods with `if logger`
    conditionals.
  - Provide the perf improvement by default, rather
    than only if `debug_logger` is set to `nil`.
Fix spec so it can run in isolation.
Always include request/response details in error.

…even when the debug logger is disabled.
Merge pull request #328 from vcr/improve_logger_perf

Improve logger perf