Added PFFileDataStream for streaming file Downloads.

[richardjrossiii]

This is an NSInputStream proxy that is used for file downloads while concurrently reading and writing to a file. This allows us to truly stream from parse using -[PFFile getDataDownloadStreamInBackground], whereas before it had the potential to break if you consumed it too fast.

[nlutsenko]

I don't remember precisely, but I think these annotations are simply ignored for class documentation.

[nlutsenko]

Looks great, minus nits.
Don't really have a good name for this class, sorry... You can name it anything, but please prefix with PF.

The only question about this that I have - what is the expected behavior for using this stream with getBuffer:length:? It will simply call through into the actual stream, just wondering what are the things people can run into.

[richardjrossiii]

getBuffer:length: is not implemented (and probably will never be) for the built-in NSInputStream classes, and the docs aren't clear how it should be used in any case (e.g. whether this buffer already has contents, or whether you should call -read:maxLength: after getting the buffer).

According to this mailing list it would appear that you should still call -read:maxLength:, but according to this, it would appear you should not.

Seeing as apple has never used this themselves and probably never will due to the API confusion surrounding it, I think it's safe enough to ignore this for now.

[nlutsenko]

Sounds great! Fix nits and let's ship!

[nlutsenko]

Please rename before shipping. As much as I love the name (it has my name in it :D), not sure if other people will understand this.