Add more unit tests

With the addition of XCTest [to the project] today, we can focus on adding more unit tests to this project to aid in contributions and future pull requests. 

## Test coverage:

### RSColorPickerView:

- [x] setSelectionColor
- [x] setCropToCircle
- [x] setSelection
- [x] setShowLoupe
- [x] colorAtPoint

### RSColorPickerViewDelegate:

- [x] Need to move this to separate file (currently tested with RSColorPickerView)
- [x] RSColorPickerViewDelegate-colorPickerDidChangeSelection
    - Don't care about the other 2 so much. 

### RSColorPickerState:

- [x] hue
- [x] saturation
- [x] color
    - All 4 init methods tested here, and this is the method that matters most
- [x] selectionLocationWithSize:padding:

### RSColorFunctions:

- [x] RSPixelFromHSV
- [x] RSHSVFromPixel
- [x] RSGetComponentsForColor
    - Made sure to test all color space possibilities
- ~~RSUIImageWithScale~~ *how to test?*
- ~~RSOpacityBackgroundImage~~ *how to test?*
- [x] RSRandomColorOpaque

### BGRSLoupeLayer:

- TODO, maybe?

### RSSelectionLayer:

- TODO, maybe?