Move from erlang.mk to rebar3

[Euen]

Migrate to rebar3 as described here.

This is requested by esl/TODO#62

[hcs42]

@elbrujohalcon @Euen The swagger example still uses erlang.mk, and when I build it, it still uses the Makefile from cowboy-swagger. That Makefile it outdated, because it refers to an old version of trails.

So when I build the cowboy-swagger example, it uses trails 0.1.1 instead of 0.2.0. This could be fixed by modifying the Makefile above. Shall I create a PR about that?

Also, this ticket is related: #50 I think it can be closed, since the swagger example doesn't hardcode the trails version any more (the swagger Makefile does...)