Fixed iteration to work in Python 2.x and Python 3.x

This change prevents the following error in Python 3.2:

```python
  File "colortuple/colortuple.py", line 167, in <module>
    for name, value in Color.__dict__.iteritems()
AttributeError: 'dict_proxy' object has no attribute 'iteritems'
```

and the following error in Python 3.3 and 3.4:

```python
  File "./colortuple/colortuple.py", line 167, in <module>
    for name, value in Color.__dict__.iteritems()
AttributeError: 'mappingproxy' object has no attribute 'iteritems'
```