Feature: Support explicitly choosing TLS version to negotiate with remote side by respecting crypto_method context parameter for all classes. (#149 by @clue)

By default, all connector and server classes support TLSv1.0+ and exclude support for legacy SSLv2/SSLv3. As of PHP 5.6+ you can also explicitly choose the TLS version you want to negotiate with the remote side:

// new: now supports 'crypto_method` context parameter for all classes
$connector = new Connector($loop, array(
    'tls' => array(
        'crypto_method' => STREAM_CRYPTO_METHOD_TLSv1_2_CLIENT
    )
));

Minor internal clean up to unify class imports (#148 by @clue)