Support for union types

First thank you for making this, great tool to get GoLang clients up to speed fast.

I noticed that union types are ignored when generating:
```
union AorB = A | B
```
In the output AorB does not exist, and other types which use it comes up as `undefined` like this:
```
type Results struct {
	Hits  *[]undefined `json:"hits,omitempty"`
	Total Int          `json:"total"`
}
```