added mix neotoma task and mix.exs for elixir

`.peg` files can be compiled  to Erlang by running `mix compile.neotoma`.

`mix.exs` is used instead of `project.exs` so that mix will be used as the build tool and the neotoma task will be compiled. The downside is that there are now configs for two different build tools, mix and rebar. Luckily, the rebar config seems to be primarily for testing. I added a `VERSION` file so that the version number can be shared among the tools -- `neotoma.app.config` can be altered to reference that file rather than `git`. I can make that change if you'd like.

I also added package info for the Elixir package repo, [hex.pm](https://hex.pm/), but I wasn't sure if the info was correct, or if you'd like to upload it yourself.

Let me know if you're considering merging this pull request and I can add a "usage with elixir" note to the documentation.

Thanks!