fix typo in setup.py


Doing this:

    virtualenv ~/newve
    git clone https://github.com/jmcorgan/bip32utils
    ~/newve/bin/pip install ./bip32utils
    ~/newve/bin/bip32gen --help

gave me this:

    ImportError: No module named ecdsa

I manually fixed it by:

    ~/newve/bin/pip install ecdsa

I think the reason it was happening was the `ecdsa` module was misspelled in `setup.py`.  This patch fixes the typo.  I haven't actually tested whether it fixes the above `ImportError` on a new install, though.