Using C++11 features and added missing asserts

[GermanAizek]

@miloyip Hi, I think you will like features. These are different overloads and they will be called in different ways.

GenericValue& operator[](SizeType index)

const GenericValue& operator[](SizeType index) const

[tencent-adm]

All committers have signed the CLA.

[sembooooo]

In case if you havent find out what the problem is , may be , my finding can help you out.

Environment: APPVEYOR_BUILD_WORKER_IMAGE=Visual Studio 2013, VS_VERSION=10 2010, VS_PLATFORM=win32, CXX11=OFF, CXX17=OFF, MEMBERSMAP=OFF; Configuration: Debug
1 min 8 sec

I have taken this from one of the build logs. default is not present in older versions of c++. It is present from c++11 and we can see that CXX11 = OFF. Hope this helps.

[pagict]

since old MSVC compilers don't fully support C++11, so the CI need to disable C++11 by default

[sembooooo]

since old MSVC compilers don't fully support C++11, so the CI need to disable C++11 by default

So are these changes applicable ? For this to be considered i think few more changes need to be done but then again is it worth the maintenance ?