Support for incoming attachments.

1) Refactored MMS layer to use abstracted types.

2) Added support for retrieving attachment IDs.

library/src/org/whispersystems/textsecure/push/PushAttachmentData.java

@@ -0,0 +1,21 @@
+package org.whispersystems.textsecure.push;
+
+public class PushAttachmentData {
+
+  private final String contentType;
+  private final byte[] data;
+
+  public PushAttachmentData(String contentType, byte[] data) {
+    this.contentType = contentType;
+    this.data        = data;
+  }
+
+  public String getContentType() {
+    return contentType;
+  }
+
+  public byte[] getData() {
+    return data;
+  }
+
+}