Rename to Serenity

This changes internal class names, as well as the default output directory to serenity.

The only thing left is properties. They still start with thucydides. Wasn't sure how we want to handle this. Just change them to start with serenity, but that would cause unknown/unexpected behaviour. Although the same can be said for moving the default output directory.

Perhaps move everything and add it to release notes so that users know if they upgrade to newer version, they'll expect a new output directory, and new properties to override it?

Thoughts?