Add harmony function.

Added harmony method to API. Tests for all cases provided in the tester.js file. Based off the link provided in the issues section (goo.gl/R3FRlU).

Examples.

I want to generate the two analogous colours either side of my chosen colour:
```javascript
var analogous = Spectra('#ff00ff').harmony('analogous', 1);
```
The first argument, ```'analgous'``` is fairly self explanatory. The second, ```1``` indicates the index in which the original color appears in the resulting list of harmonious colors. Calling with analogous will produce two analogous colors and the original color. Indicating that the original color be at the first index, it will then generate the two colors to the left (zeroth index) and the right (second index). If I were to specify 0 or 2, it will produce two analogous colors to the left or to the right. This indexing system works for all types.