Merge pull request #15 from danalloway/danalloway-provide-async

provide async implementation

Author: sagikazarmark

composer.json

@@ -22,6 +22,7 @@
         "henrikbjorn/phpspec-code-coverage" : "^1.0"
     },
     "provide": {
+        "php-http/async-client-implementation": "1.0",
         "php-http/client-implementation": "1.0"
     },
     "autoload": {