Support for URL instead of NSURL about sdwebimagephotosplugin

[dreampiggy]

This is limit in current Pure Objective-C implementation...

That NSURL category return type can not been autoamtically converted into URL sturct and may loss information.

[dreampiggy]

Can the Fix-it with as URL? solve the issue ?

[aheze]

@dreampiggy yes, as URL? works.

let asset: PHAsset
let photosURL = NSURL.sd_URL(with: asset)
cell.imageView.sd_setImage(with: photosURL as URL?, placeholderImage: nil) /// this works

[dreampiggy]

I'll update readme now.

For better Swift support, let me find some other way, or provide something like SDWebImage/SDWebImage#2980

Provide this repo as an Objective-C/Swift Mixed project.

[dreampiggy]

@aheze Introduce the convenience API for Swift user. See #21

let asset: PHAsset
let url = asset.sd_URLRepresentation