Fixes to moving-average

1. Do not require a window stream to be specified when constructing a moving average stream. Instead, create them internally based on input arguments
2. Do not inherit from Window; this is unnecessary.
3. Do not create or require a window when using the 'exponential' type, as the point of an EWMA is to not require keeping a window in memory.
4. Allow specifying 'type' instead of string 'average', if desired.
5. Add tests for moving-average.js