Write spec for Sliding Window Log class

[Stephan Halarewicz]

With the FixedWindow algorithm, users are able to send more requests to go through at the edges of a window. The SlidingWindowLog algorithm addresses this issue by tracking request timestamps in in a log then removing these requests from the log once they fall outside of the window. If a request is received and there are more than capacity requests in the log then the request is dropped.

A spec for the SlidingWindowLog algorithm should be decided on prior to writing tests. The constructor should accept parameters for the following:

windowSize - Size of the window in milliseconds
capacity - number of requests allowed during each window
client - redis client instance

The SlidingWindowLog class must implement RateLimiter