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.

isso/utils/__init__.py

@@ -56,15 +56,31 @@ class Bloomfilter:
     of space efficiency (array is saved for each comment) and 11 hash functions
     because of best overall false-positive rate in that range.
 
+    >>> bf = Bloomfilter()
+    >>> bf.add("127.0.0.1")
+    >>> not any(map(bf.__contains__, ("1.2.%i.4" for i in range(256))))
+    True
+
+    >>> bf = Bloomfilter()
+    >>> for i in range(256):
+    ...     bf.add("1.2.%i.4" % i)
+    ...
+    >>> len(bf)
+    256
+    >>> "1.2.3.4" in bf
+    True
+    >>> "127.0.0.1" in bf
+    False
+
     -- via Raymond Hettinger
        http://code.activestate.com/recipes/577684-bloom-filter/
     """
 
-    def __init__(self, array=bytearray(256), elements=0, iterable=()):
-        self.array = array
+    def __init__(self, array=None, elements=0, iterable=()):
+        self.array = array or bytearray(256)
         self.elements = elements
         self.k = 11
-        self.m = len(array) * 8
+        self.m = len(self.array) * 8
 
         for item in iterable:
             self.add(item)