Add clang-format rules

Summary:
The rule file is forked from that in Facebook's repo.

I'll add format file for now and team members can tune the rules later.

In this patch, I made only two changes in order to be consistent with existing coding style

`SpacesBeforeTrailingComments: 2`

`ColumnLimit:     80`

Test Plan: N/A

Reviewers: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15015

.clang-format

+# Complete list of style options can be found at: 
+# http://clang.llvm.org/docs/ClangFormatStyleOptions.html
+---
+BasedOnStyle: Google
+AccessModifierOffset: -1
+ConstructorInitializerIndentWidth: 4
+AlignEscapedNewlinesLeft: true
+AlignTrailingComments: true
+AllowAllParametersOfDeclarationOnNextLine: true
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakTemplateDeclarations: true
+AlwaysBreakBeforeMultilineStrings: true
+BreakBeforeBinaryOperators: false
+BreakConstructorInitializersBeforeComma: false
+BinPackParameters: false
+ColumnLimit:     80
+ConstructorInitializerAllOnOneLineOrOnePerLine: true
+DerivePointerBinding: true
+ExperimentalAutoDetectBinPacking: true
+IndentCaseLabels: false
+MaxEmptyLinesToKeep: 1
+NamespaceIndentation: None
+ObjCSpaceBeforeProtocolList: false
+PenaltyBreakBeforeFirstCallParameter: 10
+PenaltyBreakComment: 60
+PenaltyBreakString: 1000
+PenaltyBreakFirstLessLess: 20
+PenaltyExcessCharacter: 1000000
+PenaltyReturnTypeOnItsOwnLine: 200
+PointerBindsToType: true
+SpacesBeforeTrailingComments: 2
+Cpp11BracedListStyle: true
+Standard:        Cpp11
+IndentWidth:     2
+TabWidth:        8
+UseTab:          Never
+BreakBeforeBraces: Attach
+IndentFunctionDeclarationAfterType: false
+SpacesInParentheses: false
+SpacesInAngles: false
+SpaceInEmptyParentheses: false
+SpacesInCStyleCastParentheses: false
+SpaceAfterControlStatementKeyword: true
+SpaceBeforeAssignmentOperators: true
+ContinuationIndentWidth: 4
+...