Add support for Graphite web sigmoid function

[carrieedwards]

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

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

sigmoid(seriesList)
Takes one metric or a wildcard seriesList and applies the sigmoid function 1 / (1 + exp(-x)) to each datapoint.

Example:

&target=sigmoid(Server.instance01.threads.busy)
&target=sigmoid(Server.instance*.threads.busy)

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