[BUGFIX] Implement double ext key conversion to fully ensure proper format

Before: extension keys given already as "UpperCamelCase" would be converted by t3lib_div::underscoredToUpperCamelCase into "Uppercamelcase" which naturally results in an invalid extension key called "uppercamelcase" instead of the desired "upper_camel_case". By implementing a precursory conversion this case can be completely prevented.
[BUGFIX] Do not iterate over fields unless fields are actually used
[TASK] Cleanup in Preview class to use getPreparedExposedTemplateView
[BUGFIX] Present friendlier error in fallback template structure

Prevents an infinite nesting level recursion from using var_export on what are potentially recursive (Extbase) objects.
[TASK] Cleanup resulting in performance improvement

This cleans up some unused variables, switches some semi-duplicated code over to new API methods and implements an additional cache when reading configurations directly from Flux template files - resulting in big performance improvements especially on large page trees.
Merge pull request #91 from NamelessCoder/extkeyconversionfix

[BUGFIX] Unpredictable UpperCamelCase-to-UpperCamelCase conversion