combiner: combine two tables via join or extend

This implements

- an INNER join
- extend (append rows from table2 to table1)

Would love your feedback on

1. If join / extend are in scope for optimus
2. What interfaces are most appropriate for these methods
3. Suggested optimizations. Currently, this is a naive implementation, so I would like to optimize it substantially and better utilize golang 

Thanks!

![image](https://cloud.githubusercontent.com/assets/102242/3275109/b95cec6a-f337-11e3-90d5-c1c75433202d.png)