Address review feedback and clean up issues found by hh_client.
  * Rename Process::start so as to avoid a method override
  * Be more fussy about nullables.
  * Use float, not double.
  * Shorten names to allow for rational < 80 column lines.
  * Use Class::static_function() invocations.

Rewrite the option consumer to allow for multiple instances
of the flag --hhvm-extra-arguments, as poorlyl handled by getopt.

Construct and make a default tempDir as part of handling options.