Add compatibility for JsonSerializable

Fixes #24 for 5.2/5.3.
Fix /pages parent link base route
Remove redundant WP_JSON_Pages::type_archive_link

As it is identical to WP_JSON_CustomPostType::type_archive_link()
avoids 'featured_image: null' for post types that does not support thumbnail
changes hook plugins_loaded to wp_json_server_before_serve. Issue: http://wordpress.org/support/topic/extending-wp_json_customposttype-gives
Merge pull request #43 from phh/master

Thumbnails for post types that does'nt support and docs update
Use interface_exists instead of class_exists
Revised get_avatar() public method to use core get_avatar function and return stripped url.
Revised get_avatar() protected method to use core get_avatar function and return stripped url.
Added whitespace for readability to public method get_avatar().
Merge pull request #47 from rachelbaker/master

First pass at issue #35.  Updated get_avatar methods to use the core get_avatar function and extract url.
Merge pull request #24 from WP-API/jsonserializable

Introduce support for JsonSerializable
Ensure that the author is set if passed

Fixes #44
Clarify the usage of classes in plugins

Fixes #45
Ensure JSON disabled error messages are translated

Fixes #38
Removed extra "Link: " from link headers - issue #30 - https://github.com/WP-API/WP-API/issues/30 Fixes /WP-API/WP-API#30
Merge pull request #56 from jmusal/30

Remove extra "Link:", fixes #30
Merge pull request #38 from WP-API/audit-translations

Audit human-readable strings
Add 0.6-0.7 comparison link to changelog
Merge branch 'pages-link-fix' of github.com:thenbrent/WP-API