Add proper configuration for running the unit tests

Document my target of PHP 5.3 while I'm at it.

composer.json

@@ -16,6 +16,9 @@
 		"classmap": ["src/"]
 	},
 	"require-dev": {
-		"satooshi/php-coveralls": "dev-master"
+		"phpunit/phpunit": "^4.8"
+	},
+	"require": {
+		"php": ">5.3"
 	}
 }

phpunit.xml

@@ -0,0 +1,14 @@
+<phpunit
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/5.1/phpunit.xsd"
+        bootstrap="tests/bootstrap.php"
+        backupGlobals="true"
+        verbose="true"
+        colors="true"
+>
+    <testsuites>
+        <testsuite name="SimpleXML Debug Tests">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>