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
Now this url currently is flexible. It can just be a charm name 'mysql' and we'll make sure to build the series from the outer segment of the bundle definition and come up with /trusty/mysql which is valid. Is the series name in here as part of this?