Add Value::XXXMember(...) overloads for std::string

[miloyip]

Try to replace the PR #333
Would like to invite @pah to review.

[coveralls]

Coverage increased (+0.0%) to 99.82% when pulling 2786103 on member_stdstring into 7562d02 on master.

[pah]

As mentioned in #333, these functions have been omitted from the original std::string support (#107).
It is a good idea to add them now. For completeness, we should add PushBack overloads as well, though.

One of the main reasons for omitting AddMember( ..., const std::string&, ... ) is the explosion of the overload set if we want to support std::string as a key parameter as well. But this will likely pop up soon, too. ;-)

LGTM, otherwise. 👍

[miloyip]

Actually I have tried adding some overloads for AddMember(const std::string&, ...) but it will causes ambiguity error for some AddMember(const Ch*, ...) calls. PushBack() also have similar problem.
So I think this is it.