🐛 Fix missing execution permission bit in execFile override

Consider an electron application that uses `execFile` to run a script
that lives within the application code base:

```coffee
child_process = require 'child_process'
child_process.execFile __dirname + '/script.sh', (error) ->
  throw error if error?
```

An application like this will fail when being packaged in an `asar` with
an following error:

```
Error: spawn EACCES
```

Electron overrides certain `fs` functions to make them work within an
`asar` package. In the case of `execFile`, the file to be executed is
extracted from the `asar` package into a temporary file and ran from
there.

The problem is that during the extraction, the original permissions of
the file are lost.

We workaround this by:

1. Extending `asar.stat` to return whether a file is executable or not,
  which is information that's already saved in the `asar` header.

2. Setting execution permissions on the extracted file if the above
  property holds true.

Fixes: https://github.com/atom/electron/issues/3512

atom/common/api/atom_api_asar.cc

@@ -54,6 +54,7 @@ class Archive : public mate::Wrappable {
     mate::Dictionary dict(isolate, v8::Object::New(isolate));
     dict.Set("size", stats.size);
     dict.Set("offset", stats.offset);
+    dict.Set("executable", stats.executable);
     dict.Set("isFile", stats.is_file);
     dict.Set("isDirectory", stats.is_directory);
     dict.Set("isLink", stats.is_link);