Update demo app: rebar deps, ensure inets is running, improve README

Updated demo app to be more intuitive for those relatively new to Erlang and Webmachine. 

Demo wouldn't compile with rebar.config webmachine dependency set to 1.7.* - updated to 1.9.*

Fixed error due to inets not running when starting demo app.

Added a couple concrete steps to README file to test the basic app and the file resource module.