Move check for unique violations to base class CxxReportSensor: saveUniqueViolation.
Design decision: Create new method saveUniqueViolation to give derived classes the chance to call saveUniqueViolation or saveViolation.
Add saveUniqueViolation to all from CxxReportSensor derived sensors.
Add test for duplicated external rule violations.

Conflicts:
	sonar-cxx-plugin/src/main/java/org/sonar/plugins/cxx/compiler/CxxCompilerSensor.java
	sonar-cxx-plugin/src/main/java/org/sonar/plugins/cxx/cppcheck/CxxCppCheckSensor.java
merge with master & fix remaining issues
Merge pull request #148 from guwirth/bug/duplicated-violations

bug/duplicated-violations