Renames gittip to gratipay

I've done most of the work, but you'll need to do two things:

1. Merge this PR
2. Rename the repository to `gratipay-badge`.

Note that the rename will not affect existing URLs, as all web traffic is [redirected after a rename](https://help.github.com/articles/renaming-a-repository). I've also taken the liberty to shift to the rawgit CDN url, which is supposed to be [used in production](https://rawgit.com/). Also, I've kept the old gittip.png in the repo for the rare chance that someone might be using an unversioned direct link to master.

The links in README won't work till you merge this PR. I am not sure, but it will still break if the tag 0.3.1 is not merged. So you might need to tag 0.3.1 manually.
It looks like we are going to rename/relocate this into the @graitpay organization (see #5). I am not sure how well GitHub plays with multiple renames, can we move these URLs back to `gittip-badge` for now?