Add .travis.osx.yml and update .travis.yml to trigger OSX build
Include OS X build status in README.md
Use lowercases in src/CMakeLists.txt
Use numeric_traits<mpfp>::get_mpfp_rnd() as a default rounding mode in mpfp operations
Fix empty interval
Add binary operators between interval<T> and T
Add more tests to interval
Fix OSX trigger in .travis.yml
Remove OSX Trigger from .travis.yml. It will be done by other way around.