Added MagickWand.GetImageFloats() (+tests) for returning float pixel data

I create this extra method on `MagickWand` because I had a need to retrieve a "packed" `[]float32` of `RGBRGB...` to supply to `OpenColorIO`'s [Processor::apply()](http://opencolorio.org/developers/api/OpenColorIO.html#Processor::apply__ImageDescRC) method. 
It accepts float pixel information in order to apply color transformations: http://opencolorio.org/developers/api/OpenColorIO.html#packedimagedesc

After originally writing the function in Go, I saw that there was a lot of overhead in doing so many cgo calls in a loop. So I moved this routine down into C, in the imagick library. It about a 7x speed up. I structured it in a way to  hopefully make it flexible

```
func BenchmarkFloatPixels(b *testing.B) {
	wand := imagick.NewMagickWand()
	defer wand.Destroy()

	wand.ReadImage("logo:")
	wand.ScaleImage(1024, 1024)

	var (
		rowNum int
		inc    int
		pixel  *imagick.PixelWand
		pixels []float32
	)

	b.ResetTimer()

	for i := 0; i < b.N; i++ {

		inc = 0
		cols := int(wand.GetImageWidth())
		rows := int(wand.GetImageHeight())
		pixels := make([]float32, cols*rows*3)

		pixIter := wand.NewPixelIterator()
		defer pixIter.Destroy()

		for rowNum = 0; rowNum < rows; rowNum++ {
			for _, pixel = range pixIter.GetNextIteratorRow() {
				pixels[inc] = float32(pixel.GetRed())
				pixels[inc+1] = float32(pixel.GetGreen())
				pixels[inc+2] = float32(pixel.GetBlue())
				inc += 3
			}
		}
	}

	b.StopTimer()
	b.Logf("len(pixels) == %d", len(pixels))
}

func BenchmarkFloatPixelsC(b *testing.B) {
	wand := imagick.NewMagickWand()
	defer wand.Destroy()

	wand.ReadImage("logo:")
	wand.ScaleImage(1024, 1024)

	var pixels []float32

	b.ResetTimer()

	for i := 0; i < b.N; i++ {
		pixels = wand.GetImageFloats(imagick.FLOAT_FORMAT_RGB)
	}

	b.StopTimer()
	b.Logf("len(pixels) == %d", len(pixels))
}
```

Results:
```
BenchmarkFloatPixels	       2	 640257007 ns/op
--- BENCH: BenchmarkFloatPixels
	image_test.go:45: len(pixels) == 0
	image_test.go:45: len(pixels) == 0
BenchmarkFloatPixelsC	      20	  91631297 ns/op
--- BENCH: BenchmarkFloatPixelsC
	image_test.go:64: len(pixels) == 3145728
	image_test.go:64: len(pixels) == 3145728
ok  	github.com/justinfx/opencolorigo	4.426s
```