Sepia takes a parameter for amount

## The Change

This change makes allows Sepia to accept a paramter for how much it is applied.

## Demo

Here's an unaltered images:

![original](https://cloud.githubusercontent.com/assets/18454/3667687/5996e01e-120a-11e4-8e92-65b50febd882.jpg)

And here's that image going through 0 to 100 in steps of 10:

![sepia-demo](https://cloud.githubusercontent.com/assets/18454/3667688/5cfdeda6-120a-11e4-89e4-848b1ec021dd.gif)

## Why

[CamanJS](https://github.com/meltingice/CamanJS) has some really nice [filters](http://camanjs.com/examples/) that I would like to port over to go to see how performance compares.  Many of these filters make use of sepia with a paramter for how much it should be applied.