Add PATCH-based update

This simplifies the way a document can be updated. It changes the syntax to, for example:

``` sh
$ curl -XPATCH ":9200/foo/user/1" -d '{"first_name":"Stephen"}'
```

Instead of the more verbosely-nested:

``` sh
$ curl -XPOST ":9200/foo/user/1/_update" -d '{"doc":{"first_name":"Stephen"}}'
```

Adding PATCH support is a relatively big change for the project, but I wanted to at least start a discussion. I think the request format for the update example is cleaner than nesting the update in a "doc" node (and more predictable/consistent with the PUT method).