Add StackLinesContent, reimplement TracebackContent to use this new class

Add a new content class: testtools.content.StackLinesContent that takes a list of pre-processed stack lines, and produces a content object. Refactor TracebackContent class to be a function that returns an instance of StackLinesContent.

This is a precursor to adding non-fatal exceptions, since we need a way to add stack line content objects without having an exception.