Add Python binding

I've added a Python binding to HashPump, so that it can be used directly.

It is available [on PyPI](https://pypi.python.org/pypi/hashpumpy/1.0) and can be installed via `$ pip install hashpumpy`.

Here's an example:

```shell
user@ubuntu:~$ pip install hashpumpy
Downloading/unpacking hashpumpy
...
Successfully installed hashpumpy
Cleaning up...
```
```python
>>> import hashpumpy
>>> help(hashpumpy.hashpump)
Help on built-in function hashpump in module hashpumpy:

hashpump(...)
    hashpump(hexdigest, original_data, data_to_add, key_length) -> (digest, message)
    
    Arguments:
        hexdigest(str):      Hex-encoded result of hashing key + original_data.
        original_data(str):  Known data used to get the hash result hexdigest.
        data_to_add(str):    Data to append
        key_length(int):     Length of unknown data prepended to the hash
    
    Returns:
        A tuple containing the new hex digest and the new message.
>>> hashpumpy.hashpump('ffffffff', 'original_data', 'data_to_add', len('KEYKEYKEY'))
('e3c4a05f', 'original_datadata_to_add')
```
```shell
$ hashpump --signature ffffffff --data original_data --additional data_to_add --keylength 9
e3c4a05f
original_datadata_to_add
```