Replace NSURLConnection (deprecated) with NSURLSession

Infrastructure:

• Had to update the iOS deployment target to 7.0 from 5.0 - 6545a3a

Features:

• Replace deprecated NSURLConnection with NSURLSession #1578 #1586 - fixes #1291 #1318 #823 #1566 #1515
• Allow to customise cache and image downloader instances used with SDWebImageManager 86fc47b - fixes #1398 #870

Fixes:

• Removed the URL query params from the filename (key) fb0cdb6 1bf62d4 #1584 - fixes #1433 #1553 #1583 #1585
• Fixed the WebP build with the official 1.0.0 CocoaPods release f1a471e - fixes #1444
• Updated doc: removeImageForKey: not synchronous e6e5c51 - fixes #1379 #1415