benchmark: clean up common.js

This commit cleans up `benchmark/common.js` with a few generic changes
such as the following:

- declare all `require()`'d libraries at the top instead of in the
  middle
- add some empty whitespace where it helps readability
- changes ambiguous variable names
- standardizes most if / else blocks
- missing semicolons

PR-URL: https://github.com/iojs/io.js/pull/662


Reviewed-By: Nicu Micleușanu <micnic90@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>