Allow pushing of a single object to Wombat

This allows a single object to be pushed to Wombat, rather than all object of that class. It takes a class name and an ID and uses the same behaviour of `push_batches`, except the "batch" is a single object. This might be useful if you want to push things to Wombat immediately, rather than using a scheduler.

I also noticed there were specs for the client at all, so I added some in.