Add version field to BaseMessage

[MHHukiewitz]

From the discussion about pay-as-you-go specs in #82 (comment):

The chain used for payment should be the same as the chain of the sender and the receiver. The chain field of the BaseMessage class is not signed however.

Then I'd just recommend changing the verification_buffer function to include the chain field (and also a new version field describing the message spec version)

[hoh]

It would only make sense to add a version number if and when we decide to introduce breaking changes, which is not the case so far.