Travis-CI and multi-platform CI

[psi29a]

http://docs.travis-ci.com/user/osx-ci-environment/
^-- as an example

Soon there will be other platforms too, we should take advantage of this.

[corristo]

I'd be happy to use this, but Travis still cannot into multiplatform configuration in one .travis.yml, i.e. I can't just add OS X build script to our main repo's .travis.yml without breaking linux build.

I could use separate branch with edited .travis.yml for OS X, of course, but it involves manual merging =\

Related Travis issue: https://github.com/travis-ci/travis-ci/issues/216