Added deletion to custom attributes

This follows from enabling tag deletion, however does require some
consideration with the APIs.

Now we can remove custom attributes, as well as determine if an attribute
exists or not.

tests/corrupt.py

@@ -19,10 +19,11 @@ def corrupt(block):
                 break
 
             tag ^= ntag
-            file.seek(tag & 0xfff, os.SEEK_CUR)
+            size = (tag & 0xfff) if (tag & 0xfff) != 0xfff else 0
+            file.seek(size, os.SEEK_CUR)
 
         # lob off last 3 bytes
-        file.seek(-((tag & 0xfff) + 3), os.SEEK_CUR)
+        file.seek(-(size + 3), os.SEEK_CUR)
         file.truncate()
 
 def main(args):