Implemented logic from issue #36

Link to issue #36 
For the record, here's the breakdown of that logic:

* If $ETC_DIR contains **both** an app.config and a vm.args, do nothing

* If $ETC_DIR contains an app.config and **no vm.args**, generate both, but start riak using the existing app.config and generated vm.args

* If $ETC_DIR contains a vm.args file and **no app.config**, generate both, but start riak using the existing vm.args and the generated app.config.

* If $ETC_DIR contains **neither app.config nor vm.args**, generate both and start riak with both.

thanks to @jaredmorrow and @evanmcc for reporting the issue and talking it out.