Establish some rules for handling InputStreams

* Methods shouldn't place any special requirements on the streams
  passed into them
* This implies that if a stream's going to be marked and reset,
  that should all happen within one method
* This also implies that if a method needs to mark and reset a stream,
  it should wrap the stream in a BufferedInputStream before doing so,
  rather than requiring a markable stream to be passed in

briar-core/src/main/java/org/briarproject/briar/messaging/MessagingManagerImpl.java

@@ -33,6 +33,7 @@ import org.briarproject.briar.api.messaging.event.PrivateMessageReceivedEvent;
 import org.briarproject.briar.client.ConversationClientImpl;
 
 import java.io.ByteArrayInputStream;
+import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -154,8 +155,9 @@ class MessagingManagerImpl extends ConversationClientImpl
 
 	@Override
 	public AttachmentHeader addLocalAttachment(GroupId groupId, long timestamp,
-			String contentType, InputStream is) {
+			String contentType, InputStream is) throws IOException {
 		// TODO add real implementation
+		if (is.available() == 0) throw new IOException();
 		byte[] b = new byte[MessageId.LENGTH];
 		new Random().nextBytes(b);
 		return new AttachmentHeader(new MessageId(b), "image/png");