advagg 8.x-3.0-rc2

All tests converted from SimpleTest -> PHPUnit Kernel and Functional tests.
Some minor bug fixes (thanks StryKaiser for a report and patch)
Start of a rewrite of the README.
Initial implementation of CSS Selector Limiter for 8.x-3.x

Full changelog:

Changes since 8.x-3.0-rc1:

• Improve README.md
• Initial implementation of CSS Selector limiter for 8.x-3.x
• Update the asset array with the (possibly) modified array after optimizing the array for both CSS & JS assets.
• Allow subscribers to AssetOptimizationEvents to change the asset array.
• Update tests to removed deprecated asserts (mostly assert[No]Raw)
• Convert simpletest->PHPUnit: CdnTest
• Convert simpletest->PHPUnit: AdminPagesTest and all tests that extend it.
• Convert simpletest->PHPUnit: AdvaggPathManipulationTest
• Convert simpletest->PHPUnit tests: AdvaggTestBase -> AdvaggFunctionalTestBase
• Add .csslintrc to exclude test css files.
• Fix CSS minifier's handling of charset declarations.
• Css minification tests
• Ignore test JS files and included JS files from vendors when linting.
• Standards fixes.
• Don't test JSMin minification if the JSMin extension is unavailable.
• #2888100 by StryKaizer, NickWilde: jsqueeze licensing information is inverted
• Test JsMinifier::optimize() when minifier set to JShrink
• Test JsMinifier::optimize() when minifier set to JSqueeze.
• Test JsMinifier::optimize() when minifier set to JSMinPlus
• Test JsMinifier::optimize() when minifier set to JSMin.
• Test advagg_js_minify JsMinifier::clean()
• Fairly complete testing of the CDN sub module.
• Install functionality for advagg_cdn to clear caches so it just works on install.