Adds tests on class BlobDetection. Fixes #31.

For #31, a std::set was used prior to the addition of outline points to
the outline vector, so as to forbid duplicates.

	modified:   pandora_vision_hole_detector/src/utils/blob_detection.cpp
	modified:   pandora_vision_hole_detector/test/CMakeLists.txt
	new file:   pandora_vision_hole_detector/test/unit/utils/blob_detection_test.cpp