Add union type support
Add —import option to specify import headers
Include GLKit and OpenGLES framework by default
Make sure to import framework headers for custom class

- generate framework import headers from metabase.sytem_frameworks_map.
- you still need to specify `—imports` if there’s no framework info in the map

```
--frameworks=OpenGLES --frameworks=GLKit --imports="OpenGLES/EAGL"
```
remove debug log
Merge pull request #41 from infosia/union_type

Add union type support