Add mobileOnly section for label and mode to the Dynamic Preferences example.

When SmartApps use dynamic pages, label and mode properties are not
added automatically to the app's preferences. This change adds the
fields to the Dynamic Preferences example. Initially I was going to
call this a bug, but seeing how the official apps do this, I think
at the very least it needs to be documented.
Merge pull request #16 from sudarkoff/feature/dynamicPrefExample

Whenever Dynamic Preferences are used in a SmartApp, the section for label and mode is not added to the app's preferences automatically. This change add that section to the example code.