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().
Updated plugin activation and deactivation functions to be multisite compatible.

Check if the plugin is network activated in Multisite, and if so run activation or deactivation function for each public blog in the network.  Initial pass at issue #48.
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
Added conditional to WP_JSON_Posts->prepare_author() to verify the $user object is set.
Fixes bugs with attaching featured images to posts

This commit fixes two bugs in `WP_JSON_Media::attachThubmnail()`.

    - `attachThumbnail()` should dp nothing if `$update` is false since
    the post ID is needed.
    - The post ID must be fetched from the `$post` array.
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