Add zkutils package

[nemith]

Java provides a JKUtil class with a bunch of static methods for doing common operations with zookeeper. I think we should follow this and keep the client expectations that one call to a method generates a message to the server and not try to overload this.

We can add things like #52

[nemith]

Potential methods to add:

DeleteRecusive(zk *zk.Client, path string, size int) error  
DeleteBatch(zk *zk.Client, paths []string, size int) error
Tree(zk *zk.Client, path string) []string, error
Walk(zk *zk.Client, path string, func(path string) error) error 

[linsite]

I'm interested, can I take this task?