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
This should be more informative, for example:

```go
logger.WithField("error", err).Error("unable to process job synchronously")
```