Makes git metadata available to build

Git SHA-1 of `HEAD` and branch name are incorporated into:
* manifests as Implementation-SHA-1 and Implementation-Branch
* REST API at `/v1/server/version`.
* jsgui (only sha-1) as comment in index.html

I've modified `ServerApi.getVersion` to return a `VersionSummary` object rather than just a String:
```
$ http :8081/v1/server/version
HTTP/1.1 200 OK
Content-Encoding: gzip
Content-Type: application/json
Server: Jetty(8.1.4.v20120524)
Transfer-Encoding: chunked

{
    "buildBranch": "build-sha1",
    "buildSha1": "63e9f1ec8864fa2ac5ccd1f99f28a7f0b260a8b4",
    "version": "0.7.0-SNAPSHOT"
}
```

Should I deprecate the old version and add a separate API call instead? There were no uses of the method in Brooklyn.