Remove basic authentication handler

Also removes the json_check_authentication filter. All authentication
handlers should switch to the determine_current_user filter added in
https://core.trac.wordpress.org/changeset/27484

Adds a json_authentication_errors filter to allow reporting a WP_Error
back to the API on failed authentication.
Remove basic authentication test

This test will intentionally fail now that basic auth has been removed.
Initial pass at user endpoint documentation.
Move current user docs to after get single user
Add more verbose documentation for /users/me
Document available user data contexts
Slightly reword and rewrap deletion reassign docs
Add missing underscore to WP_User_Query
Document the /posts/<id> context parameter
Document the /posts context parameter
Document multiple post type support for /posts
Note default contexts
Reword "array" input syntax documentation
Add documentation on discovery
Syntax highlight JSON examples
Note that we use pluralised forms of nouns in routes
Merge pull request #176 from WP-API/getting-started-docs-update

Update getting started documentation.  Closes #176.
Merge pull request #175 from WP-API/compatibility-docs-update

Improve and clarify "array" input syntax documentation.  Closes #175 and #140.
Merge pull request #174 from WP-API/post-docs-update

Update post routes documentation.  Closes #174 and #172.
Merge pull request #158 from WP-API/docs-user

Add Documentation for User Endpoints.  Closes #158.