boost::variant_alternative<size_t I, boost::variant<Types...>> implem…

[very-cool-name]

…entation. Uses variadic templates for modern compilers and boost::mpl magic for older ones. Tested on gcc 4.9.3, 6.3.0 and msvc 14.1. I'll add documentation little bit later.

[apolukhin]

I'm afraid this won't work for variants that were created using make_variant_over. I'm also not sure that this will work for variants that hold a reference type.

Please add more tests.

[very-cool-name]

There is test for make_variant_over it is called test_over_sequence_type. Is it insufficient? I'll add tests for reference types.