Saving to file, other improvements

[atomicbird]

I've added a few features and made some improvements to ZSURLConnectionDelegate:

• Incoming data is saved to a file instead of being held in memory. If filePath has been set then it is used, otherwise a temporary file is created and later cleaned up.
• File writes are done asynchronously on a GCD queue while the network connection is still running.
• Self-signed certificates can now be accepted either from all hosts or from a specific list of hosts.
• Can be initialized with an NSURLRequest instead of an NSURL, so that request headers can be set (old initializers are still present).
• New convenience initializers.
• Optional userInfo property.

This also changes the asset manager demo app, fixing memory issues and getting it to load photos found in the RSS feed and display thumbnails in the table view.