Header/footer picture support

This is a basic implementation of header/footer picture support allowing to insert jpeg images into headers/footers of the xls sheets as long as &G is present in the relevant positions. A partial implementation of the "something ugly" part from [this](https://groups.google.com/forum/#!topic/python-excel/3hZP_hK_LSc) thread (where the last deleted message used to be mine, I think).

Not very polished or efficient (generates a new record for every sheet/position even if the images are identical) but it works, so, I thought it might be worth a pull request.

Most usage and implementation info is in hfpicture.py comments.