doc(coding_style.md): add section about git pre-push hook

[skip ci]
doc(README.md): use horizontal layout for the badges

simplified the title (i.e. from Ubuntu 12.04 LTS => Ubuntu), however, the detailed information are still accessible by positioning mouse over the icons.

[skip ci]
doc(README.md): "Code coverage" => "Coverage" to fit in one line

[skip ci]
style(lua): fix cpplint.py warnings

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
style(lua): fix cpplint.py warnings