fixed ImportError bug

when you try to import pygit2 in a python shell you the following ImportError:

```bash
$ python
Python 3.3.0 (default, Dec 22 2012, 21:02:07)
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pygit2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.3/site-packages/pygit2/__init__.py", line 34, in <module>
    from repository import Repository
ImportError: No module named 'repository'
```
This patch fixes the nameing issue.