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
we should have some other charm values that should pass

    charm: mysql
    charm: trusty/mediawiki
    charm: mysql-10 
    charm: trusty/mediawiki-10