Refactored Language package

This PR refactors the full Language package to become an object-oriented set of classes and refactors the unit test suite.

A new `LanguageHelper` class is created with some of the static methods that used to exist in the `Language` class.

The `_` method is deprecated in the `Language` and `Text` classes in favor of a `translate` method in order to provide clear method names.

New Service Provider classes are included in the package for use with Joomla's DI Package to store a `Language` and `Text` object to the DI Container.  The `LanguageProvider` has a dependency on a config object being a `Joomla\Registry\Registry` instance with three parameters: `language.basedir`, `language.default`, and `language.debug`.

Unit tests are refactored to test smaller units of code.  In the current state, code coverage is 98.89%
(710 of 718 lines covered).

Notes about the changes are included in the README file.