U2fval

** Major version: This release is NOT backwards compatible! See
doc/Migrating_From_Older_Versions.adoc for details.
** Now implements the U2FVAL API version 2, which is aligned with the FIDO
U2F JavaScript API 1.1 and offers more control over operations.
** Now using Flask.
** Python 3 support has been improved.
** New features:
- Challenges can now be specified by the caller.
- Properties can be set when generating a register/authenticate request,
which will be stored on completion of the request.
- Which devices to include in an authentication can be specified by the
caller.

** Fixed broken cachetools import.
** Fix HTTP responses under Python 3 not specifying a charset.
** Drop Python 2.6 support.

** Added support for Python 3 (Python 2 still supported).
** Uses python-u2flib-server-4.0.0, which replaces M2Crypto with Cryptography.
** Added transports to Device entries.
** Added DB migration command, to migrate an older database to the latest
version (see doc/Database_Setup.adoc for details).

** Fixed bug with entrypoint for CLI tool.
** Only install configuration files if the user has permission to do so
to enable installing without root).
** Fixed argument parsing when <name> is given immediately after --facets.

** Public beta release.