tutorial.cpp: fix insitu parsing (closes #273)

The insitu parsing example in the tutorial is using a local char
buffer inside a nested scope.  After leaving the scope, the
resulting Document object still references this (now invalid)
memory locations.

Some compilers reuse this space on the stack for other local
variables later in the function, corrupting the original document.

Drop the local scope to extend the lifetime of the buffer.

document.h: include <string>, iff RAPIDJSON_HAS_STDSTRING==1

Reported-by: Janusz Chorko (@yachoor)
See: https://github.com/miloyip/rapidjson/commit/c1c9ba7c#commitcomment-10434694

Check libstdc++ version to decide if enable rvalue ref support.

.travis.yml: disable SSE4.2 on 32-bit (closes #266)

Exclude intermediate files from install of in-source build. (Issue #259)

Fix VS2012 compilation fail in stringbuffertest

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