Use pretty json format for metadata.json

Chef suggests using `knife diff` for comparing local repo and chef server.

To compare cookbooks uploaded with ridley we need to compile metadata.json first.

There's already a knife command for it:
 `knife cookbook metadata from file metadata.rb`

It generates metadata.json using pretty json format.
So I suggest generating pretty jsons for metadata.rb when uploading so we can use `knife diff` against it and see nice diffs.

Note:
There's still a difference between ridley's and knife's JSON.pretty_generate since knife is using https://github.com/opscode/ffi-yajl.
`yajl/json_gem` adds an extra new line in empty hashes and `json` doesn't:
```ruby
[1]pry(main)> require 'yajl/json_gem'
true
[2]pry(main)> JSON.pretty_generate(a: {})
"{\n  \"a\": {\n\n  }\n}"

[3]pry(main)> require 'json'
true
[4]pry(main)> JSON.pretty_generate(a: {})
"{\n  \"a\": {\n  }\n}"
```
Not really sure how to deal with the difference in how json and yajl's json are generating jsons. Only found one difference so far, might be not a big problem. Added an issue for yajl-ruby as well: https://github.com/brianmario/yajl-ruby/issues/150