Add rebar3 support

[leostera]

Hello! opened this as a first stab and to get the ball rolling.

It currently seems to work fine, if I point at it using

{deps, [ {rebar, {git, "https://github.com/ostera/lagra", {ref, "master"}}} ]}.

Let me know if everything still works on your end :)

[darkling]

Looks reasonable, and works for compiling, as far as I can tell. I haven't tried actually running anything from the rebar3-built version yet. :)

Three points, I think:

• does the rebar.lock file have to be committed? It looks to me like it should be in .gitignore instead.
• rebar3 ct doesn't work -- probably because I've put the ct tests in ./ct, not in ./test. I think it needs some configuration in the test profile in rebar.config. Can you sort that out?
• The ./src/lagra.app.src file is incorrect (it's got the wrong version, for a start), and doesn't play well with erlang.mk -- if you could respin this patch without that file in, I'll commit the ./ebin/lagra.app file which erlang.mk builds. I've checked with Tristan, and he says that should be sufficient to make everything work smoothly.

Oh, and building the edoc is broken, but I think that's expected behaviour right now, because building it with erlang.mk is broken in the exact same way (oops).