Add support for basic type pointer fields

Allows models like:

```
type User struct {
	Id    int64
	Name  string
	Email *string `orm:"null"`
}
```

This helps a lot when doing JSON marshalling/unmarshalling.

Pointer fields should always be declared with the NULL orm tag for sanity, this
probably requires documentation.
Merge pull request #762 from pdf/pointer_field_support

Add support for basic type pointer fields