Fix #13

- Now correctly use signed variant of types instead of unsigned
- Use mod() instead of "%" (see
http://stackoverflow.com/questions/12276675/modulus-with-negative-numbers-in-c)
Removing clang support. See #11
Adding autobuild of SFML and SFGUI for Travis
Included cmath for gcc compilation
Adding some unittests
Merge pull request #15 from Viq111/layer-negative-fix

Layer negative fix
Merge branch 'master'