Engine(-Tests): recursive html parsing used same TextMessagePartModel in multiple places

when setting the text of a specific TextMessagePartModel, the text somewhere else in the message also changed
now there are unit tests checking for such behavior
Engine(-Twitter): Refactored newline normalization into MessageBuilder

Conflicts:
	src/Engine/Messages/MessageBuilder.cs
Engine: erasing newlines caused words to be stuck together of HTML messages
Merge branch 'stable'

Conflicts:
	src/Engine-Twitter/Protocols/Twitter/TwitterMessageBuilder.cs
	src/Engine/Messages/MessageBuilder.cs