fixed import and docu issues

* fixed ImportError bug: when you try to import pygit2 in a python shell you get 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.

* fix docu generation
   * fixed imports for docu generation
   * use low level api if there is no equivalent high level one
   * added c-ext entry for low level api