support cctld .sx
Change tests library

change YAML reader to symfony/yaml, and twitter-text-conformance will be
fetched using composer.
remove submodule
remove README
remove unuse const in phpunit.xml
Twitter_Extractor::extractRepliedUsernames() check end_mention_match
extractURLs check invalid short domain
extractURLs In the case of t.co URLs, don't allow additional path characters
extractURLs more correctly