Added 'remove_fields' method and changed 'as_marc' in record.py

Hi Ed and everyone,
I've been wandering here for some days now and I think that pymarc can be useful for me. I ran into two small problems and now I'm posting the fixes I found (I'm using Python3.4).
First, I added remove_fields method to the class Record, to eliminate all the fields with certain tags (it operates similar to a get_fields nemesis) without all the loops needed by the remove_field method.
Secondly, I found that the as_marc method modified the instance attribute 'leader' from str to bytes, not only for the function's output but for the Record object also, corrupting it. So I separate 'self.leader', that remains a text string, from the output of the function. Is this a fix o am I missing something?