To contribute a bugfix or new feature please fork the GitHub repository and push your changes to your own tree. When you think your code is ready and stable create a pull request in the main repository to have your changes merged.
Please make sure to only propose one feature or bug fix per pull request. Mixing a lot of stuff into one pull request needlessly complicates and delays handling. Also take a look at the coding standard.
Always write a clear log message for your commits. One-line messages are fine for small changes, but bigger changes should include a separate description.
Before pouring serious work into some new feature it may be a good idea to let us know via an issue on Github. This way you avoid duplicating work or pouring effort into something that has little chance to be accepted in the main tree.
You can contact @botic directly on Twitter or via Github.