add clang-format to constraint the incoming new commits .

generally all the new added codes must be follow this style guildline.
the developers are encourage to use various IDE tool to adapt this format style.
Modern IDEs are usually support this .clang-format.

.clang-format

@@ -0,0 +1,18 @@
+# Format Style Options - Created with Clang Power Tools
+---
+AllowShortFunctionsOnASingleLine: None
+AllowShortIfStatementsOnASingleLine: true
+BasedOnStyle: LLVM
+BinPackArguments: false
+BinPackParameters: false
+BreakBeforeBraces: Allman
+BreakConstructorInitializers: AfterColon
+ColumnLimit: 160
+ConstructorInitializerAllOnOneLineOrOnePerLine: true
+ConstructorInitializerIndentWidth : 2
+ContinuationIndentWidth: 2
+MaxEmptyLinesToKeep: 2
+PointerAlignment: Middle
+SpacesInAngles: true
+SpacesInParentheses: true
+...