charm: add BundleData, parser and Verify method.

This is the start of the bundle specification discussed at https://docs.google.com/document/d/1SF8hTBi6oVbki8V__beNij6wnQU-5cm6PZsy5gf0j_Y
other formats to test and verify work are the current yaml list formats such as the bundle https://manage.jujucharms.com/api/3/bundle/~charmers/hadoop/7/cluster/file/bundles.yaml