Support for pulling prerequisites described in a cpanfile.

Hi Jeff,

Here's a first cut at support for reading prerequisites from a cpanfile.

I ended up slipping it into the Pull action, it seemed to be minimally disruptive and I couldn't bring myself to cut-and-paste the rest of the Pull stuff.

Added a test to make sure that handling no targets and no cpanfile does the right thing and added a bunch of tests for various cpanfile monkeyshines.  I'm not entirely sure that I'm using the test stuff to its fullest potential.

Comments, please.