Added utility for generating test coverage reports

Added utility for generating test coverage reports using gcov and lcov

The report is created using the added ./gencoverage.sh utility, which uses lcov to
create an easy to understand line coverage report html-page.
    README.md
    common_build.py
    devtools.sh
    gencoverage.sh   (+)
    src/SConscript
    src/SConscript_tests