Support Swift

[zhouhao27]

Add Swift language.

[jonongjs]

Hi @zhouhao27
Thanks for contributing! Can I make a few suggestions?

• It would be great if you could perform some cleanup, like removing the trailing whitespace introduced in the C++ section of README.md, and also removing extraneous or user-specific files as specified in Swift.gitignore (I'm looking at .DS_Store and stuff in xcuserdata/).
• This is optional, but you can consider turning this into a Swift package as demonstrated on raywenderlich.com.