dillon1000/react

Add rudimentary jsc perf-counters runner

Works at least on a CentOS 7 machine after running `sudo yum install
webkitgtk webkitgtk-devel`.

The only globals you get are `print` and `PerfCounters`. No `console` nor the other globals provided by the `jsc` command-line tool (load, readFile, etc) though they're probably not hard to implement.

You can disable the JIT by setting the environment variable `JSC_useJIT=false`.

Test Plan:
```
~/local/react/scripts/perf-counters$ make
~/local/react/scripts/perf-counters$ build/jsc-perf <(echo 'PerfCounters.init(); var a = PerfCounters.getCounters().instructions; print("moo"); var b = PerfCounters.getCounters().instructions; print(b - a);')
moo
72182
~/local/react/scripts/perf-counters$
```