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()
Use interface_exists instead of class_exists
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.
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
Merge pull request #45 from WP-API/clarify-class-usage

Clarify usage for plugins
Merge pull request #44 from WP-API/post-insert-author

Post Author != logged in user Check in insert_post wrong...
Merge branch 'remove-type_archive_link' of github.com:thenbrent/WP-API
Remove redundant get_avatar function

See #35