Merge pull request #10 from bchretien/master

Add NLopt install script.
.travis/build: look for cov-build independently of the toolkit version number
build: enable verbose mode
build: add coverity path properly
after_failure: display environment variables.
coveralls: forward build_dir to coveralls.
Add coveralls log for debug.
Avoid error when source file cannot be found.
Add PaGMO install script.