Support loading apps in asar format

Fixes #251.

* [x] Provide a tool to create asar packages https://github.com/atom/asar
* [x] Add `asar://` protocol to load files in asar packages
* [x] Make Node's API work with asar packages
  * [x] fs.readFile
  * [x] fs.lstat
  * [ ] fs.stat
  * [x] fs.exists
  * [x] fs.readdir
  * [x] fs.open
  * [x] fs.realpath
  * [x] process.dlopen
  * [x] child_process.fork
  * [x] child_process.execFile
  * [x] native module
* [x] Support loading apps in asar package
* [x] Unit tests
* [x] Docs
  * [ ] Performance
  * [x] `cwd` option of some APIs
* [x] Testing on Windows