[swift] String To MD5

• 我將這個Function寫在extension裡面,首先Swift要能用MD5的function必須先建立header檔並掛上(header檔如何建立請看這篇) ```swift
  #import <CommonCrypto/CommonCrypto.h>

*   然後 ```swift
func md5() -> String! {
        let str = self.cStringUsingEncoding(NSUTF8StringEncoding)
        let strLen = CUnsignedInt(self.lengthOfBytesUsingEncoding(NSUTF8StringEncoding))
        let digestLen = Int(CC_MD5_DIGEST_LENGTH)
        let result = UnsafeMutablePointer<CUnsignedChar>.alloc(digestLen)
        CC_MD5(str!, strLen, result)
        var hash = NSMutableString()
        for i in 0..<digestLen {
            hash.appendFormat("%02x", result[i])
        }
        result.destroy()
        return String(format: hash)}