Replace HTTPClient with Net::HTTP

@opscode/release-engineers this is a sample PR for what it would look like to remove HTTPClient in favor of `Net:HTTP` (stdlib). The only thing that's missing is multipart forms for upload, which is only required for the config object (and will correspond to the one failing test).

This also removes I18n and standardizes errors. 

I personally like this approach a bit more, and it's also much much faster. I believe it also indirectly fixes #4 and #8.