do not reference bytearray in a method definition, fixes #5

This is a severe issue which makes the current voters bloomfilter
completely useless. Functions are first-class objects in Python, which
lead to interesting "issues" like:

    >>> def foo(x=[]):
    ...     x.append(1)
    ...     print x
    ...

    >>> foo()
    [1]

    >>> foo()
    [1, 1]

For Isso, this means the bloomfilter, which is usually only initialized
with the author's IP address, is now initialized with pretty much all
ip addresses from previous authors, thus makes it impossible for the
author to vote on other's people comments.
add database migration

Clear voters bloomfilter and initialize with an (almost) empty one.
Merge branch 'fix/bloomfilter'