Skip running perftest on CI debug builds (Appveyer; Travis)

GenericValue::AddMember<T>: add missing overload (closes #254)

RapidJSON is C++, need to set CMAKE_CXX_FLAGS instead
of CMAKE_C_FLAGS when configuring the build environment.

(Accidentally dropped in 40c03114)

As discovered by @felipegb94, there are missing overloads to the
`GenericValue::AddMember<T>` template function, taking an explicit
`GenericValue&` as a name and accepting arbitrary primitive values.

This patch adds the missing overloads. The `StringRefType` overload
is needed to disambiguate the addition of a string literal as
value.

Some tests are added to `TEST(Value, Object)` in `valuetest.cpp`.

RapidJSON.pc.in: include dir not set

Variable in wrong format is not replaced

Compilation fixes

readme.md: improve build status rendering by Doxygen

unittest.h: make sure to #include <stdexcept>

See https://github.com/miloyip/rapidjson/commit/a2a0d161#commitcomment-9838110

Doxygen currently does not support images with links, which breaks the
current rendering of the front page at https://miloyip.github.io/rapidjson.

Move the status badges to a table with the links in the top-row instead.

While at it, add a description below the logo and put the GitHub and
documentation links into in itemisation.

error.h: drop trailing comma in enum

In C++'98/03, trailing commas in enumerations are not allowed, but have
been introduced in C++11. This patch drops the trailing commas in order
to avoid compiler warnings (e.g. GCC with -pedantic).

See #9 and http://code.google.com/p/rapidjson/issues/detail?id=49 for
previous instances of this issue.

add AppVeyor CI support for checked Windows builds

Move `gtest_force_shared_crt` option to Win32 VC only.