Rename Gittip to Gratipay

This branch will rename Gittip to Gratipay as per [this discussion](https://github.com/gittip/building.gittip.com/issues/73). It is based on the i18n branch. The first commit was just a simple search and replace:

    git grep -l 'Gittip' | xargs sed -i 's/Gittip/Gratipay/g'