Merge pull request #25 from barryvdh/patch-1

Markdown is not required, so move to suggest

composer.json

@@ -6,8 +6,7 @@
         {"name": "Mike van Riel", "email": "[email protected]"}
     ],
     "require": {
-        "php": ">=5.3.3",
-        "dflydev/markdown": "1.0.*"
+        "php": ">=5.3.3"
     },
     "autoload": {
         "psr-0": {"phpDocumentor": ["src/"]}
@@ -15,6 +14,9 @@
     "require-dev": {
         "phpunit/phpunit": "3.7.*@stable"
     },
+    "suggest": {
+        "dflydev/markdown": "1.0.*"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "2.0.x-dev"