Add sliding windows
Sliding Window in-place updates rather than providing a new window each time
Finally sort everything out

- EventStream.slidingWindow(n) returns an EventStream
- Property.slidingWindow(n) returns a Property

Currently I'm unable to test that it's the same array every time,
but that should be the case.
Edits are no longer in place (as d3 can cope without), and .slidingWindow(n) always returns an EventStream
Example uses .scan() to be more bacony
Merge branch 'master' into sliding-windows
Implement sliding windows in terms of scan

- hence it now returns a Property rather than an EventStream, I hope you're keeping track
Implement slidingWindow in Observable
Merge pull request #45 from lenary/sliding-windows

Added Sliding Windows