Allowing summation of Seq, SeqRecord and Alignment objects?

Hello all, this pull requests includes zero as a special case in the `__radd__` methods of these classes (invoked by Python when doing `0 + my_seq` or `0 + my_record` etc) which happens inside a summation call (e.g. `sum([seq1, seq2, seq3])`.

I recently I had to write something like this:

    # Some loop to build up a combined alignment
    merged_alignment = None
    for x in y:
        if z:
            # create some new alignment object w, then...
            if merged_alignment is None:
                merged_alignment = w
            else:
                merged_alignment += w

This does not feel elegant, the natural style was to sum a list:

    # Some loop to build up a list of alignments to be combined
    alignments = []
    for x in y:
        if z:
            # create some new alignment object w, then...
            alignments.append(w)
    merged_alignment = sum(alignments)

An interesting side effect of this is you can do things like this:

    from Bio import SeqIO
    merged_record = sum(SeqIO.parse(filename, format))

Which seems interesting. Overall I'm unsure if this is a compelling improvement or not... thoughts?