removed @covers annotations
minor #1571 removed @covers annotations (fabpot)

This PR was merged into the 1.16-dev branch.

Discussion
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removed @covers annotations

I propose to remove the `@covers` annotations for the following main reasons:

 * they were only used in a limited number of test cases;
 * they limited the code coverage of nodes for no reasons (some other tests are as valid to test the nodes).

Commits
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4ba0789 removed @covers annotations
bumped version to 2.0.0-DEV
removed deprecated Twig_Filter_* classes
removed deprecated Twig_Function_* classes
removed deprecated Twig_Test_* classes
emptied deprecated interfaces
updated CHANGELOG
removed deprecated token parser broker
removed deprecated Twig_Environment::removeExtension()
improved the performance of the filesystem loader
removed the ability to register a global variable after the runtime or the extensions have been initialized
simplified code
simplified code
simplified code
fixed a unit test as Twig_ExistsLoaderInterface has been merged into Twig_LoaderInterface
remove unneeded string cast
remove string casts from loader which was done for template reference objects, but this type of parameter is not contracted by the LoaderInterface
added missing deprecation notices
removed deprecated tests