Add initial work for building a/b tested projects

In order to make R.JS work we've removed the 'tests' config option and
hard-coded the test list to 'tests.js' (next to config.js). This is definitely
not great; this file should be customisable as either an object or string as
per issue #1
Fuck it, let's use hard-coded dependencies

Override them using path and map config settings. Jobs a goodun.