charmstore: add CORS headers to all requests
Merge pull request #200 from rogpeppe/086-CORS

charmstore: add CORS headers to all requests

To QA this, we need to create a web page that contains some javascript
that makes an AJAX request to the charm store, and check that it
succeeds.
internal/storetesting: add YAMLEquals checker
Merge pull request #166 from rogpeppe/073-yaml-equals

internal/storetesting: add YAMLEquals checker

We want this for checking YAML output elsewhere.