Support Interfaces for schemaForGoType

[AtakanColak]

Hiya, I came across "interface types (interface {}) not yet supported (use avrogo instead)" error whilst trying out this library. Error comes from gotype.go#273. Maybe interface{} types could be matched with type casting? I want to be able to get schemas for maps like below;

myMap := map[string]interface{}{ "time": time.Now(), "value1": 1, "tag1": "test1", }