Skip comments while reading plist/dict/array

Without this patch there are problems reading this:

```xml
        <key>CFBundleName</key>
        <string>Emacs</string>
        <key>CFBundlePackageType</key>
        <string>APPL</string>
        <!-- This should be the emacs version number. --> *** get "undefined" key here and further keys are messed up.
        <key>CFBundleShortVersionString</key>
        <string>24.3</string>
        <key>CFBundleSignature</key>
        <string>EMAx</string>
        <!-- This SHOULD be a build number. -->
        <key>CFBundleVersion</key>
        <string>9.0</string>
```