[WIP] Add option to cancel a build

~~this is a rough draft, I'm actively working on it.. so it may throw errors and it may have dirty code.~~

   - Add cancelling and cancelled status for builds
   - Remove pod and update builds to cancelled when a cancel event was triggered for the build
You should log this error with `glog`, `fmt.Errorf` will create an error object but nothing is going to happen with it, since the `HandleBuild` method does not return anything.