Add travis clang-format job

Author: awegrzyn

.clang-format

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+BasedOnStyle: Google
+AccessModifierOffset: -1
+AlignEscapedNewlinesLeft: true
+AlignTrailingComments: true
+AllowAllParametersOfDeclarationOnNextLine: false
+AllowShortFunctionsOnASingleLine: true
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+#AlwaysBreakBeforeMultilineStrings: true
+AlwaysBreakTemplateDeclarations: true
+BinPackParameters: true
+BreakBeforeBinaryOperators: false
+BreakBeforeBraces: Linux
+BreakBeforeTernaryOperators: true
+BreakConstructorInitializersBeforeComma: false
+ColumnLimit: 0
+CommentPragmas:  '^ IWYU pragma:'
+ConstructorInitializerAllOnOneLineOrOnePerLine: true
+ConstructorInitializerIndentWidth: 2
+ContinuationIndentWidth: 2
+Cpp11BracedListStyle: false
+DerivePointerBinding: false
+ExperimentalAutoDetectBinPacking: false
+IndentCaseLabels: true
+IndentFunctionDeclarationAfterType: true
+IndentWidth:     2
+# It is broken on windows. Breaks all #include "header.h"
+Language:        Cpp
+MaxEmptyLinesToKeep: 1
+KeepEmptyLinesAtTheStartOfBlocks: true
+NamespaceIndentation: None
+ObjCSpaceAfterProperty: false
+ObjCSpaceBeforeProtocolList: false
+PenaltyBreakBeforeFirstCallParameter: 1
+PenaltyBreakComment: 300
+PenaltyBreakFirstLessLess: 120
+PenaltyBreakString: 1000
+PenaltyExcessCharacter: 1000000
+PenaltyReturnTypeOnItsOwnLine: 200
+SortIncludes: false
+SpaceBeforeAssignmentOperators: true
+SpaceBeforeParens: ControlStatements
+SpaceInEmptyParentheses: false
+SpacesBeforeTrailingComments: 1
+SpacesInAngles:  false
+SpacesInContainerLiterals: true
+SpacesInCStyleCastParentheses: false
+SpacesInParentheses: false
+Standard:        Cpp11
+TabWidth:        2
+UseTab:          Never

.travis.yml

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+language: cpp
+matrix:
+  include:
+    - os: linux
+      dist: xenial
+      env: TOOL=clang-format
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-xenial-7
+          packages:
+            - clang-format-7
+      compiler: clang
+script:
+  - if [[ $TOOL == "clang-format" ]] && [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
+      cd $TRAVIS_BUILD_DIR;
+      BASE_COMMIT=$(git rev-parse $TRAVIS_BRANCH);
+      COMMIT_FILES=$(git diff --name-only $BASE_COMMIT | grep -i -v LinkDef);
+      RESULT_OUTPUT="$(git-clang-format-7 --commit $BASE_COMMIT --diff --binary `which clang-format-7` $COMMIT_FILES)";
+      if [ "$RESULT_OUTPUT" == "no modified files to format" ] || [ "$RESULT_OUTPUT" == "clang-format did not modify any files" ] ; then
+        exit 0;
+      else
+        echo "$RESULT_OUTPUT";
+        exit 1;
+      fi
+    fi
+notifications:
+  email: false