Cascading - almost ready for merge

Had a slightly different idea for this, but I'm totally open to changing if you think it doesn't work...

I was thinking that users would continue to use the common operators defined in `pigpen.core`. Then for pig I added `pigpen.pig/generate-script` and `pigpen.pig/write-script`. I was thinking that you would add `pigpen.cascading/generate-flow` and put the stuff you have into a `pigpen.cascading.flow` namespace.

That way a user wouldn't have to change any code to target a different platform - only the final command to change it to use something else.

Thoughts?