R15 callbacks

[jlouis]

What title says. Provides Callbacks in R15 style so they can be dialyzer-checked.

[jlouis]

I added something so we now require R15.

Yes, this breaks backwards compatibility, so we may want to not pull the request. On the other hand, I'd rather that we move on to R15 quickly :)

[uwiger]

Let me ponder this for a while. Not sure who uses jobs and if this would cause breakage. In general, I'm in favor of the change.

[jlouis]

We don't need to pull the branch right away. It shouldn't be that hard to keep up-to-date and it should help my dialyzer runs by quite a lot :P In the case you want to ponder, we may need to pull the rebar hint about erl_first_files though as this is needed to make rebar not complain about missing behaviours.