Build and publish wheels for Mac OS

[chaitan94]

While the code compiles and wheel are being built on Mac OS right now, they are really linked properly. As a result, when using the wheel to import pymeos, we see the following:

     from pymeos.box import STBox
E   ImportError: dlopen(/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmp96l8_7pd/lib/python3.5/site-packages/pymeos.cpython-35m-darwin.so, 2): Symbol not found: __ZN10SerializerI8GeometryE5writeEPK11TInstantSetIS0_E

[chaitan94]

Fixed in 3d73f33 and 0f95a15, GitHub action was added in 3a1704e.

The issue was mostly the following two:

1. Template instantiations need to happen in .cpp, not .hpp. More explanation here: https://stackoverflow.com/a/8752879/1932097
2. Mac OS's clang needs explicit #includes in every place where classes like string, stringstream etc., are used so that they are instantiated properly