Default subjects to :all when action is provided

[mikepack]

This adds support for:

can :do_something

Which is equivalent to:

can :do_something, :all

Providing an action but no subjects will currently fail silently when invoked and throw an AccessDenied exception in the controller for non-resourceful actions. Fixes #734.

[carloslopes]

@ryanb this is stuck by 8 months.. any chances to be merged or rejected?

[davidakachaos]

@ryanb ping?