Added custom header handler.

[couling]

This branch was offered by @sbluhm to implement adding custom headers (see enhancement request #9).

[sbluhm]

Let me update, test and merge with the latest changes.

[couling]

@sbluhm because this was so long I wasn't sure if you'd still be there to check it so I created it against a branch in my own repo. You can either create a duplicate PR (pulling from your fork) or you can comment here when you're done and I'll pull your changes in.

[sbluhm]

I think I was waiting for a verification of the statement before creating a pull request.

"I am not releasing the memory for xmlTextReaderGetAttribute here and have no clue how to do this. According to the documentation, you have to de-allocate it."

If you have the changes ready, feel free to go ahead with your branch.
Am watching your project as I am using it productively. Whenever I find time, I am happy to contribute.