Don't use #count for testing #drop
Implement #scan

This operations takes a zero value and a block and applies the
block to every element using the last result as the first argument
and the current element as the second. The initial computation will
use the zero value as the first argument.

This #scan implentation as the first element in the result stream
is not the zero value as it is in other situations. This is because
the zero value element occurrence 'time' wouldn't make any sense.
Implement Stepper

This is based on the `stepper` function in the original FRP
implementation that returned a Behaviour where `beh.at(t)` would
return the occurrence with the maximum occurrece time where `occ.time <=
t`.

Here we can evaluate the Stepper Behaviour at the present time by
calling #now.
Return a Stepper rather than an Integer from Stream#count
Remove @count from Stream Stream implementations
Stepper => Property
Merge pull request #11 from seadowg/master

Return a Behaviour from Stream#count