mac: Convert from ImageSkia to NSImage to reserve DPI info.
Recognize the "@2x" suffix of icon's filename.
Fix converting empty V8 dictionary.
Use mate::Dictionary instead of base::DictionaryValue for options.

mate::Dictionary can represent arbitray type, which matches our use.
Dicard uses of base::Value in native_window.
Support high dpi icon as window icon.
Fix compilation error.
:memo: Add docs on image support in atom-shell.
Wait for crash reporter spec longer.
Merge pull request #423 from atom/hidpi-icon

Add support for high resolution icon