Make sure not to use bottleneck when it is not installed.

[lodagro]

I don`t have bottleneck, this little change allows me to import trunk again
nosetests running fine

[wesm]

Sorry about that. I changed it to a slightly more EAFP:

    try:
        bn_func = getattr(bn, bn_name)
    except NameError:
        bn_func = None