Add codecov.io to the repo

[mauricioaniche]

Let's add codecov.io, which allows us to follow our coverage!

(That would had been really useful at the beginning, though. Maybe a bit less now given that we are about to deploy, but ... I think we'll code this forever hehe)

[mauricioaniche]

Ah, I'd focus on the Java project for now, and once we tackle the python one, we see what we do.