Rewrite of JSON documentation

This pull request contains a partial rewrite of the "Working with JSON" documentation for Play w/ Scala. Here's what was done:

- Rewrite of “JSON Basics” to trim noise, re-organize, polish content, fix grammatical/language errors, and improved examples (all compiled/tested). This page is more of a “quick start” and points to other pages for details on advanced patterns and concepts.
- Renamed “Handling & Serving JSON Requests” to “JSON with HTTP” and moved to immediately follow “JSON Basics." Polished page and rewrote examples to be compiled/tested and model a typical REST web service.
- Drastic rewrite of “Reads/Writes/Format combinators to cover practical use of Reads/Writes for the current version using combinators. Heavily example based (all compiled/tested), lighter on theory, etc.
- All rewritten pages share a common model and JSON for consistency.
- “JSON Transformers”, and “JSON Macro Inception” have not been modified at this point. This will either be a phase 2 of the rewrite, or be replaced by new “Validation” docs in a future release.

This fixes #2395