make escript -- rebar escriptize

Implements `make escript`, an equivalent to `rebar escriptize` but
* Uses `create/2` from STDlib's `escript` module
* Restricts executable rights to user: `chmod u+x …`
* Note: the module exporting a `main/1` function has to be the file `src/$APP.erl`
* escript > escripti[zs]e