仿照ibatis的风格实现Log，以及添加ConnectionChecker

git-svn-id: http://code.alibabatech.com/svn/druid/trunk@292 b9813039-fb51-4c41-a8b9-e21c2acb5095

src/main/java/com/alibaba/druid/logging/Log.java

@@ -0,0 +1,17 @@
+package com.alibaba.druid.logging;
+
+public interface Log {
+
+    boolean isDebugEnabled();
+
+    void error(String s, Throwable e);
+
+    void error(String s);
+
+    public void debug(String s);
+
+    public void warn(String s);
+    
+    void warn(String s, Throwable e);
+
+}