Dispatch io on keywords

Change io operations to dispatch on keywords instead of pig-specific loader classes

cc @pkozikow - Let me know if this works for you
This is where I'm dispatching on the new storage keyword to generate the pig script for that storage. Right now there's just :string and :binary, but :binary is only used by io/load-binary and io/store-binary, which aren't even included in pigpen.core.