Add include for std::free from <cstdlib>
Update bootstrap.py to handle external lua directories and linking on
linux.
Add uninstall and install targets to ninja.
Add travis-ci integration
Add --ci flag to travis-ci script
Update .travis.yml
Update travis script, fix warnings.
Ignore some warnings on clang.
Properly handle const value returns (and constness in general).
Keeps errors when trying to bind a const variables directly,
but const returns should be forced to value-types,
since lua has no concept of `const`.
Merge remote-tracking branch 'root/master'

Conflicts:
	sol/userdata.hpp