Add support for Graphite web exponentialMovingAverage function

[carrieedwards]

Is your feature request related to a problem? Please describe.
The exponentialMovingAverage() function is implemented in Graphite web, but is missing from CarbonAPI. In order to enable users to be able to use the exponentialMovingAverage() function, it needs to be implemented in CarbonAPI.

Describe the solution you'd like
The exponentialMovingAverage() function should be implemented in CarbonAPI, with the same functionality as the Graphite web function. The description of the function is as follows:

exponentialMovingAverage(seriesList, windowSize)
Takes a series of values and a window size and produces an exponential moving average utilizing the following formula:

ema(current) = constant * (Current Value) + (1 - constant) * ema(previous)
The Constant is calculated as:

constant = 2 / (windowSize + 1)
The first period EMA uses a simple moving average for its value.

Example:

&target=exponentialMovingAverage(*.transactions.count, 10)
&target=exponentialMovingAverage(*.transactions.count, '-10s')

Additional context
Tests should be added to verify that the functionality matches that of the Graphite web exponentialMovingAverage() function.