Python 3 Support

[janxious]

Using the sample code from the README

$ python3 ./dr.py
Traceback (most recent call last):
  File "./dr.py", line 13, in <module>
    "document_type": "pdf",                                         # pdf or xls or xlsx
  File "/usr/local/lib/python3.5/site-packages/docraptor/apis/doc_api.py", line 203, in create_doc
    callback=params.get('callback'))
  File "/usr/local/lib/python3.5/site-packages/docraptor/api_client.py", line 322, in call_api
    response_type, auth_settings, callback)
  File "/usr/local/lib/python3.5/site-packages/docraptor/api_client.py", line 149, in __call_api
    post_params=post_params, body=body)
  File "/usr/local/lib/python3.5/site-packages/docraptor/api_client.py", line 358, in request
    body=body)
  File "/usr/local/lib/python3.5/site-packages/docraptor/rest.py", line 208, in POST
    body=body)
  File "/usr/local/lib/python3.5/site-packages/docraptor/rest.py", line 171, in request
    r.data = r.data.decode('utf8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe2 in position 10: invalid continuation byte