Implement building blocks of multi-key transactions

[bonifaido]

rust-zookeeper would needs transaction support first though (#4)? This correctly we'd need something similar to the Java client's multi method, and possibly something like the Transaction builder - which seems like a wrapper on top of multi. That would also involve creating something similar to Op, and finally, support for serializing and deserializing the multi requests and responses - similar to MultiTransactionRecord and MultiResponse.

[tgockel]

I have a branch for this here: https://github.com/tgockel/rust-zookeeper/tree/issue/45/transactions . The code doesn't work because I can't get the deserialization of responses working correctly and I have no idea what I'm doing wrong :-(