Unit tests

This adds unit tests into Tatoeba. There are fixtures for most tables, along with some tests I used to fix #60. This also fixes a few missing includes (App::import()).  Running the site usually includes tons of files, so the missing includes were included anyway and we didn’t notice that problem. Merging this branch into master will allow new features to be developped using [test driven development](https://en.wikipedia.org/wiki/Test_driven_development), and show regressions.