README: Removes source map from stats.
Keep sourceMap as string

Fixes [object Object] being written to source map file.
Merge pull request #583 from didoarellano/fix-source-map-output

Fix [object Object] being written to source map file
Code: Improves outFile condition.
Code: Moves temp return after the condition.
So manual build can kick in.
Binding: Passes ctx_w pointer to helpers.
* Pushes NanDisposePersistent to very end.
Code: Caches result as soon as v8 returns.
Not fixes segfault bug, but brings clarity of intent.