Resolve "Create a script to implement account key rollover"

Closes #3 (closed) by adding an account key rollover tool inside /tools. Note that current implementation doesn't follow exactly acme-draft-05, but it follows current letsencrypt/boulder ACME server implementation.

Tests have been updated to add a little test for this tool, the acme_account_delete tool (which already existed for acme_dns_tiny tests and was so moved to /tools directory too). Now, scripts from this project can import tools and tests from their respective directory.