Update man page
Add preliminary header to magic file
Add functions for version checking

Add version enum and functions to parse version string or return
human-readable version string from enum.

Rename encrypted file version check function.
Update to use version enum
Update for backwards compatibility

Allow the user to specify which version of encrypt to encrypt their file
for. This can be set from the command line or the config file.

Will always default to the latest version, with all the
bells-and-whistles, but if version 2012.11 or earlier is requested, it
will actually fallback as far as possible.