py3k conversion

Hi,

I'm using this python module in my application (python3 made).

Because of python3 evolution, module quote_plus is not anymore a part of core urlib, **trello-py** is not usble is third version of python.

```python
Traceback (most recent call last):
  File "./cli.py", line 38, in <module>
    driver = load_class('manager.%s.%s.%s' % (manager, manager, class_name))
  File "./cli.py", line 17, in load_class
    module = importlib.import_module(module_path)
  File "/usr/lib64/python3.3/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1586, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1567, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1534, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 586, in _check_name_wrapper
  File "<frozen importlib._bootstrap>", line 1024, in load_module
  File "<frozen importlib._bootstrap>", line 1005, in load_module
  File "<frozen importlib._bootstrap>", line 562, in module_for_loader_wrapper
  File "<frozen importlib._bootstrap>", line 870, in _load_module
  File "<frozen importlib._bootstrap>", line 313, in _call_with_frames_removed
  File "/home/slide/workspace/tk/manager/trello/trello.py", line 2, in <module>
    from trello import TrelloApi
  File "/usr/lib/python3.3/site-packages/trello/__init__.py", line 1, in <module>
    from urllib import quote_plus
ImportError: cannot import name quote_plus
```