Review

Just to preface this, I haven't touched pretty much any of the code functionality.  Overall, it looks great.

This PR is my review over the package to get it up to standard.  It's mostly dealing with repo structure and documentation.  Just to try and summarize:

- Added `composer.lock` to the `.gitignore`, the same as the rest of the Framework packages
- Filled in all the base info for the `composer.json` file and restructured it a bit to load in testing dependencies only in the development environment
- Moved the test classes into the `Joomla\DateTime\Test` namespace
- Got all of the doc blocks up to standard with PHPCS
- Adds the coding standard as a submodule to the repo
- Renames all the Interfaces to align with the rest of the Framework's naming convention
- Improved language use in some docs
- Improved extensibility of `Date` and `DateTime` classes.