getCrop() returns whole percentages, losing sub-percent precision #36

Since the fix for #33 the crop state is kept as a % crop (onChange stores react-image-crop's percentCrop). Crop holds int values, so getCrop() rounds those percentages to whole units.

On a 4000px-wide image, 1% is 40px of granularity, and setCrop(getCrop()) visibly moves the selection. Previously onChange stored the pixel crop, so the granularity was roughly one rendered pixel.

Proposal: widen Crop's x, y, width and height from int to double. Note this changes the record's component types, so it is a breaking change for callers that read them — 1.3.0 would be the place for it.

Reported by @scardanzan while reviewing #35.