Fixed boss_db_rebar plugin to compile boss_db models before eunit

I don't know if it's just an configuration problem from my side, but otherwise it seems that it's currently not possible to unit test OTP applications with eunit which depend on boss_db/models. ```pre_compile/2``` is only executed with ```rebar compile``` hence models doesn't get compiled with ```rebar eunit```. With this fix I've implemented ```pre_eunit/2``` for the boss_db rebar plugin which compiles the models and save them to the ```.eunit``` folder instead of ```ebin```.