Summary:
@public
Add PerformanceLogger to keep track of JS download, initial script execution and
full TTI.
Test Plan:
The Native side currently calls `addTimespans` when it's finish initializing
with the six values (start and end for the three events), so I just checked it
with a `PerformanceLogger.logTimespans()` at the end of the function.
```
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "ScriptDownload: 48ms"
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "ScriptExecution: 106ms"
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "TTI: 293ms"
```
Tadeu Zagallo