make CFBundleName and CFBundleIdentifier unique

`Atom` and `com.github.atom` are already in use by the editor. Let’s
give the shell its own identity so poor OS X doesn’t get confused.

Closes #686
I think we should use `com.github.atom-shell` since `atom-shell` is the product name.

And we also need to change the string in `atom/renderer/resources/mac/Info.plist`.