General cleanup:

-> Adding Exceptions for non-existant notebooks.
-> Adding and using PopulatedSection as it's significantly faster then PopulatedSections
-> Fixing UT bugs.
-> AddingGetPageContentAsXDocument as it's running XML deserializing twice is expensive.
-> dumbTodo cleanup
Fixing SmartTagProcessUT by having Setup run for every unittest (via [Setup] vs [TestFixtureSetup]
Minor code cleanup in SmartTags.