Cleanup/fixes based on feedback

* Use accessor methods instead of instance variables for new_resource
  and current_resource in the provider.
* Use Chef::JSONCompat
* Move etc require to top
* Use let for homebrew_owner specs
* Set homebrew owner attribute and remove ENV stubs
* Add repeated stubs in LCR to a before block
* Wrap action method tests in a context so provider.current_resource
  does the right thing
* Add timeout for #get_response_from_command (bonus, per
  #opscode-cookbooks/homebrew issue 49)