Date formats, reusable cards, documentation and base64 encoding

This should not be a breaking change, but is a minor release just in case, since there are a few bits in here.

• In some instances the returned data format from Opayo contains nano-seconds that PHP 7.4 has a problem parsing. A change strips out the nano-seconds if they are present.
• Documentation is updated to describe how to encode the 3DS session data, and decode when the user returns to your site. This may be built into the package at some point, when we are sure it can be done as a non-breakign change.
• Additional features added to handle repeating cards.

Thank you all for these contributions :-)

Switch to Elavon/Opayo URLs for 2024

See issue #4

First Opayo release

3.0.0

Updated README references