add author and committer in params of the commit function

Currently, when doing a commit with GitPython, it is based solely on the parameters defined in the environment variables GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL. 
It is very painful as a way of doing things. 

This Pull Request allow to make a commit through the function commit the index class. 

With this update, we can make a commit like that.

```python
repo = Repo('my/repo/')
index = repo.index
index.add['manifest.txt']
index.commit(msg.encode('utf-8'), 
              author=Actor('firm1', 'firm1@yahoo.fr'), 
              committer=Actor('firm2', 'firm2@yahoo.fr')
            )
```
When author or committer params aren't specified, the default value is variable environment.

Results are off course in the logs entry.