Multithread disk cache write

[Dimillian]

https://github.com/Dimillian/HackerSwifter/blob/master/Hacker%20Swifter/Hacker%20Swifter/HTTP/Cache.swift#L126

Put that line in a dispatch async in a background queue. At the moment I can't because I get some odd errors with the Swift compiler... Will do in next beta.