async+perf: ch6, tweaks to wording, clarifications around jsPerf behavior quirks
async+perf: more tweaks/clarifications to ch6 'jsPerf' discussion
async+perf: ch6, more on 'jsperf'
async+perf: ch6, adding 'writing good tests'
async+perf: ch6, merging PR #263 from @Shakeyy