Add synchronous HTTP requests support

* Add configuration option for `serve` to decide whether sync or async
  builds are default. Defaults to async builds.
* Default value can be set with `--sync-build` or
  `DOCKER_BUILDER_SYNCBUILD`.
* Requests to DockerBuild webhook can override this setting on a
  job-by-job basis by adding `sync` key to JSON body.

Fixes #67
We're already unmarshaling the body a few lines down in the function.  This could be simplified by adding the `sync` field to the `JobSpec` itself instead of using a separate config struct.