remove stringstream dependency

should fix the GCC unittest

out_of_range exceptions where unexpected before - the parser used to crash in these situations...

Provide a from_json overload for std::map

This overload is chosen only when BasicJsonType::string_t
is not constructible from std::map::key_type.

Currently, converting a map to json treats it as an array of pairs.

fixes #1079

fix typo in README

The README says:
> "The library itself contains of a single header file ..."

I guess "consists of a" is the wording that was intended here. Alternatively, "contains a" might also be what was intended. Either way, I think both versions work equally well and thus choose the one that I believe was intended.

Fix typo