Add ability to provide callback url to get notified on complete of build

If a build-image request is initiated with a "CallbackUrl", then geard will reply with a 202 immediately instead of tailing the log into the HTTP response.  Upon completion of the build, geard will do a POST to the callback location provided that contains the build log.  

Example:

curl -X POST "http://localhost:43273/build-image" -H "Content-Type:  application/json" -d '{"BaseImage":"pmorie/fedora-mock", "Source":"git://github.com/pmorie/simple-html", "Tag":"mybuild-1", "CallbackUrl":"http://localhost:8000"}' -O