Gson customization docs

Added a Gson customization section to the website
Added blurb about GSON documentation
Fix identation in Gson customization docs; emphasis that Retrofit just offloads to Gson for custom conversion behavior
Update custom conversion docs per feedback

* Clarify that Gson is just the default converter
* Add section on other built-in converters
* Add section on implementing your own custom converters
Fix small wording issues in custom converter docs
Merge pull request #359 from swanson/gson-customization-docs

Gson customization docs