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.
Allow multiple post types to get_posts
Provide invalid type name in error message
Update get_posts phpDoc to match types change
Return WP_Error in get_comment for invalid comments - resolves #166
Removed unused prepare_author method from WP_JSON_Posts class. Fixes #165.
Merge pull request #165 from WP-API/remove-prepare-author

Remove unused prepare_author method from WP_JSON_Posts class
Merge pull request #163 from WP-API/multiple-post-types

Add multiple post type support to get_posts method.  Fixes #142.
Initial pass at user endpoint documentation.
Merge pull request #171 from tlovett1/bugfix/fatal-error-on-invalid-comment

Return WP_Error in get_comment for invalid comments - resolves #166
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