Adding a composer file
Adding a readme for the collections library
Added markdown link instead of using plain url in Collection readme
Adding a replace map in the main composer.json
Merge pull request #4375 from cakephp/3.0-collection-subtree

3.0 collection subtree
Added assertion for PluginTest::testIgnoreMissingFiles()
Renamed testProvider() to ExceptionProvider().
And updated docblock @return
False tests

* False risky fix - testAuthNoAuth() is never calling write.
* ControllerTestCaseTest::testTestActionWithMultipleRedirect() not asserting anything...
* SessionHelperTest::testFlashIncomplete() not asserting anything...
* ResultSetTest::testRewind() not asserting anything...
Test ResultSetTest
Test incomplete flash message
Asserting "hack" to avoid "Risky" test result.
Removed testAuthNoAuth() kind of useless.