Added the convenience API to get URL representation from PHAsset object

[dreampiggy]

This PR added one simple API for Swift user. Which just map to the NSURL category.

This close #18

Now you don't need the as URL.

[codecov]

Codecov Report

Merging #21 (2f782d4) into master (eaa2393) will increase coverage by 0.10%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   63.70%   63.80%   +0.10%     
==========================================
  Files           3        4       +1     
  Lines         270      268       -2     
==========================================
- Hits          172      171       -1     
+ Misses         98       97       -1     

Flag	Coverage Δ
iOS	63.80% <50.00%> (+0.10%)	⬆️

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Impacted Files	Coverage Δ
...tosPlugin/Classes/PHAsset+SDWebImagePhotosPlugin.m	0.00% <0.00%> (ø)
...hotosPlugin/Classes/NSURL+SDWebImagePhotosPlugin.m	89.28% <100.00%> (+8.03%)	⬆️

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