implemented optional dependencies

Plugin authors could now define optional dependencies to other plugins in their pom.
Extensions which are using classes from optional dependencies must specify this
with the "requires" attribute of the extension annotation.
Extensions with "requires" attribute are not installed if one of the specified plugins,
is not installed.

scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java

@@ -189,7 +189,7 @@ public final class PluginProcessor
 
     PluginTree pluginTree = new PluginTree(smps);
 
-    logger.trace("build plugin tree: {}", pluginTree);
+    logger.info("install plugin tree:\n{}", pluginTree);
 
     List<PluginNode> rootNodes = pluginTree.getRootNodes();