Add User endpoint (no user_meta handling)
Handle POST to create a user
Add calls to apply_filters for update and create
Fix HTTP status code usage; use constants for them

Also updates some error messages
Use absint() instead of (int) throughout
Remove redundant $type parameter to get_users
Remove unused Last-Modified code in get_user()
Fix null response from DELETE (and do what Post endpoint does)
Fix broken build (PHP 5.2.17) by moving const's into class
Check more appropriate capabilities, including meta caps

Also, make error messages less apologetic. Sorry about that.
Reformat some control statements and blocks
registerRoutes -> register_routes
Use consistent error messages

This brings permissions error messages in line with the post endpoints.
Check permissions on user creation
Rework some control flows
Remove some redundant comments
Use the result of json_pre_insert_data
Correct hook name
Add json_prepare_user filter
Don't double-check user ID

get_userdata will check if the user ID is valid, let's not second guess
this ourselves.