Generate remaining test fixtures

Make strings option by making them pointers

In Java strings can be nil and Jute uses this functionality for optional
strings.  To replicate this we can make all strings pointers.  It makes
the generate code a bit less friendly to use, but for the most part the
generate structs shouldn't be exposed to the end user so we can hide
this.

Had nil for vectors and buffers

Buffer and vectors will return a -1 (0xffffffff) for a nil so we need to
support that.

Technically strings have a nil value as well but they aren't used in the
exisiting go zk client.

Check for external modules in container types

When looking for external modules to import into the file we were
only looking at *parser.ClassTypes and not looking to see if there
is a vector or map with an included type.

This adds a recursive lookup for vector item items, and map value types.
We are currently ignoring map key types as we only allow pointers to
classes and so key values would be pretty worthless.

Since jute will only really be used by zookeeper i think this is a fine
trade off as it doesn't use maps at all.

Update README

Add usuage and examples of options as well as a warning and todo.