Fixed to work in conjunction with releases

When the OTP release mechanism is involved, `rebar_vsn_plugin` fails on `reltool.config` so that's what we've fixed here.

We also changed the autogenerated comment in the resulting `.app` file to let people know that it was generated by this plugin. Since we don't expect anybody to edit this (generated) file, Emacs mode settings are not necessary there and was removed thus.

To be honest, I originally wanted to submit pull request that contains only the first commit (a5db014) but GitHub does not allow me to make a pull-request from anything else than from the master's HEAD :-(