Add operator_info

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix typo

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
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