Set 'Access Control' headers option on container

[sausin]

As discussed in #58, CORS headers will be required on the container for the form post signature setup to work flawlessly.

Accordingly, following modifications would be good:

• A new command to set the additional meta option (at minimum Access-Control-Allow-Origin)
• The new command and the existing command to set key should confirm resetting of meta with option to keep other headers

These should be sufficient to get things going.