Add missing main()

Previously, "console_scripts" in "setup.py" was pointing to a non-existent main(). This would result in the following error:
```shell

$ cdiff
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pkg_resources.py", line 2017, in load
    entry = getattr(entry,attr)
AttributeError: 'module' object has no attribute 'main'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/myint/Library/Python/3.3/bin/cdiff", line 9, in <module>
    load_entry_point('cdiff==0.0.1', 'console_scripts', 'cdiff')()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pkg_resources.py", line 343, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pkg_resources.py", line 2308, in load_entry_point
    return ep.load()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pkg_resources.py", line 2019, in load
    raise ImportError("%r has no %r attribute" % (entry,attr))
ImportError: <module 'cdiff' from '/Users/myint/Library/Python/3.3/bin/cdiff.py'> has no 'main' attribute
```