Issue #AR-1_property_value_converters: switched to using Archetype's native GetValue<T> when getting property values,

- added umbracoSettings.config to the test project: conversion of decimal numbers, e.g. '12345.34' triggers
'NormalizeNumberDecimalSeparator' which in turn requires via 'DefaultShortStringHelper' that the umbracoSettings.config file is loaded. We added this to make sure tests deserializing decimal numbers pass.
Merge remote-tracking branch 'origin/dev/AR-2_property_value_converters' into dev/AR-1_serialization