Bundle validation: verify with charms.
Do not use the blobstore to retrieve the bundle charms.
Return the list of bundle verification errors in JSON format.
Improve comment in bundleCharms.
Improve docstrings.
Fix the bundleCharms method and provide unit tests.
Fix typo in export_test.go filename.
Minor fixes to the bundleCharms method.
Merge pull request #70 from frankban/bundle-verify-charms

Bundle validation: verify with charms.

Validate the bundle with respect to charms in the store.