Fix warnings when compiling with some -W options.

Some people (like me) have a need to enable a few extra warnings that are not enabled by default. Many projects even enforce some of these warnings flags.

These two commits avoid (or fix, if you will) a bunch of warnings caused by both `-Wno-implicit-atomic-properties` and `-Wsign-conversion`.

Commit 9ee1ff3 also adds an extra condition (and changes another) to check whether an `NSInputStream` did fail a read operation.

    +       NSInteger bytesRead = [stream read:buffer maxLength:sizeof(buffer)];
            if (bytesRead == 0) {
    +           // 0 indicates that the end of the buffer was reached.
    +           break;
    +       } else if (bytesRead < 0) {
    +           // A negative number means that the operation failed.
                break;
            }

and

            NSInteger len = [(NSInputStream *)stream read:buf maxLength:16384];
    -       if (len) {
    -           [[self PACFileData] appendBytes:(const void *)buf length:len];
    +       // Append only if something was actually read.
    +       if (len > 0) {
    +           [[self PACFileData] appendBytes:(const void *)buf length:(NSUInteger)len];
            }

Commit 4602e69 fixes some deprecated methods in iOS 6.0.