variant comparators

[matpen]

I have two enhancement suggestions regarding the variant comparators:

1. variant::operator>() seems to be missing; while it is perfectly possible to compare the other way around, it would be convenient to have it at one's disposal (along with "<=" and ">=").

2. I have read in the docs that some types might need the manual registration of the comparators... it would be nice to have the opportunity to check if that happened, so that the program can notice that the result of the comparison is actually invalid: for example by introducing "rttr::type::has_less_than_comparator()" and analogous functions.

[acki-m]

I implemented 1.

@matpen
Can you make a new issue for the 2nd task?

By the way, even if there is no comparison operator available for a given type, it might be, when the type is converted. The other things are template types like std::vector<T> they have operator< declared, but it might not work, when the underlying type T has no comparison operator implemented.

[matpen]

Thank you very much for working on this! I added #80 as you requested.
Regarding your observations: they are correct and should definitely be added to the documentation, I think.