Adding a lot of stuff that php-zip depends on

SafeBoxEncryptor/vendor/psr/http-message/README.md

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+PSR Http Message
+================
+
+This repository holds all interfaces/classes/traits related to
+[PSR-7](http://www.php-fig.org/psr/psr-7/).
+
+Note that this is not a HTTP message implementation of its own. It is merely an
+interface that describes a HTTP message. See the specification for more details.
+
+Usage
+-----
+
+We'll certainly need some stuff in here.