Git endpoint

There's work towards that on this branch:

https://github.com/inducer/relate/commits/git-endpoint

The point of this is basically that you can 'git push' directly to relate without going through a secondary repo. I'm envisioning something like

```
git push relate preview
```

to enable a preview, or

```
git push relate live
```

to update the live site. Either of those should only succeed after the commit passes validation.

This is a prerequisite for #70. (so it's possible to get work done inside relate out)

cc @zwang180