Objective-C coding conventions

This pull request:

- Improved the TSMessage class files so they (kinda) meet the **[Objective-C coding conventions](https://github.com/github/objective-c-conventions)**.
- Moved kTSMessageAnimationDuration to the .m file
- Improved .gitignore so it ignores all the Xcode stuff

You should really code using these conventions, so the potential users will understand your code better.