Added CMake Target Export to the Installation Step for easy Integration of BGSLibrary in user projects. (#171)

usage:
cmake_minimum_required(VERSION 3.5)
find_package(BGSLibrary 3.0.0 REQUIRED)
add_executable(BackgroundSubtractionTester main.cpp)
target_link_libraries(BackgroundSubtractionTester BGSLibrary::BGSLibrary)

Co-authored-by: IVI\ihrke <stephan.ihrke@ivi.fraunhofer.de>