Refactoring and bug fixes

- Avoiding warning in path, allowing no value for historical reverse route.
- Grouping imports.
- Bug fix for historical search ordering.
- Bug fix for explanation setting.
- Better error handling (NonFatal only).
Tests for HtmlEnrichments, removing cancels

- "Cancellable are but a hint". Twitter doc is right. Let's get rid of them.
- Addition of tests for tweets. Removing of " around their content.
Tailrec annotation, lazy vals

- Using lazy vals rather than vals (might be lighter at startup on
heroku).
- Tailrec calls annotation when required.