Unit tests: Set hooks network mode back to false in points test case
Points Hooks: register new points hooks when they are added

We have to also update `WordPoints_Points_Hooks::$hooks` to be a
reference so that the hooks that are registered late will be recognized.

Fixes #32
Clean up points hooks AJAX and preserve custom settings

Previously, whenever a hook was updated, all settings would be reset.
Now only the core settings (name, prefix, suffix) are set, and custom
settings are preserved.

Preparation for #29
Add AJAX unit tests for points types and hooks

Fixes #33
Clean up points hooks AJAX callback

See #33
Points types: allow 'meta_key' setting to override default key

See #29