benchmark has no specifications so won't be run by default
don't open SDL if input device is dummy
make coverage target for code coverage for the unit test
so that we can easily find which module needs unit test

In order to run 'make coverage' `luacov` need to be installed
with 'sudo luarocks install luacov'.
install luajit for luacov in Travis
add coveralls badge to show coverage number
Update README.md
Merge pull request #956 from chrox/coverage

code coverage target for the unit test