Jim Fulton pointed out that binascii's b2a_base64()
function restricts the length of its input to 57
characters for no good reason: the code would work for
any input size but for this check. Also, there are
situations where it makes sense not to append a
newline, or to append something else than a newline.

Proposal:

- get rid of the input size check

- add an optional argument giving the delimiter string
to be appended, defaulting to "\n"

- possibly special-case None as the delimiter string to
avoid adding the pad bytes too???