Refactor action_helpers into an abstract class

Instead of being a mixin module, `AutoNetwork::ActionHelpers` now forms the
core of a new `AutoNetwork::Action::Base` class that all other AutoNetwork
actions inherit from. This allows some common action definitions, such as the
initialize method, to be centralized along with the helper methods.
Add assertive accessor for the global pool_manager

Many AutoNetwork actions will fail if they try to use
`AutoNetwork.pool_manager` when it is set to nil. This patch adds an
`active_pool_manager` accessor that raises an informative error if the
setting has an unusable value.
Update UI logging

Make AutoNetwork messages clearly identifiable and restore some missing
logging.

Fixes GH-16.