Union signature

Implemented union signature. NOTE, it seems that libffi doesn't support passing unions by value, so that feature is currently missing in pyobjus. We can pass union pointer to some function, we can retrieve union value, or union pointer from some function, but we can't pass union by value. I made example of this in examples/union.py file, so users can see what they can do with unions in pyobjus. I found this issue -> https://github.com/atgreen/libffi/issues/33, and contacted author of this proposal, but he says that he isn't yet implemented that feature in libffi.