Add initializer tests and move classpath files

The initializer tests verify that every non-optional initializer, this
includes Providers, is loadable.

Creating files under META-INF is not considered best practice. Smack's
configuration and provider files reside now in classpath directory
qualified by Smack's package namespace.

extensions/src/main/java/org/jivesoftware/smackx/ExtensionsProviderInitializer.java

@@ -0,0 +1,16 @@
+package org.jivesoftware.smackx;
+
+import org.jivesoftware.smack.initializer.UrlProviderFileInitializer;
+
+/**
+ * Loads the default provider file for the Smack extensions on initialization.
+ * 
+ * @author Robin Collier
+ *
+ */
+public class ExtensionsProviderInitializer extends UrlProviderFileInitializer {
+    @Override
+    protected String getFilePath() {
+        return "classpath:org.jivesoftware.smackx/extensions.providers";
+    }
+}