Implement Leaky Bucket Algorithm

[shalarewicz]

Implement LeakyBucket class per the spec in #50. The implementation should pass the tests written in #52

The LeakyBucket algorithm works similarly to TokenBucket except that requests are processed at a fixed rate. The algorithm is usually implemented with a FIFO queue and works as follow:

• When a request arrives, the limiter checks if the queue is full. If it is not full, the request is added to the queue.
• Otherwise, the request is dropped (a 429 status is sent)
• Requests are pulled from the queue and processed at regular intervals

[shalarewicz]

Add retryAfter to rate limiter response per #89

[evanmcneely]

Include testing and spec.