Abstract base class to inherit from if you want to create streams implementing
the same api as node crypto streams.
Requires you to implement 2 methods _final and _update. _update takes a
buffer and should return a buffer, _final takes no arguments and should return
a buffer.
The constructor takes one argument and that is a string which if present switches
it into hash mode, i.e. the object you get from crypto.createHash or
crypto.createSign, this switches the name of the final method to be the string
you passed instead of final and returns this from update.
