Fix #473 clang -Weverything

Abandon RAPIDJSON_NORETURN_SUFFIX

encodings.h: fix typo in preprocessor condition (closes #494)

https://github.com/travis-ci/travis-ci/issues/5225

detect little endian for Microsoft ARM targets

When _MSC_VER and _M_ARM is defined, this is some Windows ARM target
using little-endian byte order.

use <wchar.h> with C++ linkage on Windows ARM

Instead of commenting that we should, just do include <wchar.h>
with C++ linkage when compiling for Windows on ARM.
Actually, omitting the C linkage really should be enough here.

Fix #483 by using the correct value type

fix typos and dead links in docs

GenericDocument: add implicit conversion to ParseResult

To simplify the error handling, this commit adds an implicit conversion
of GenericDocument to ParseResult, allowing code like (already in the
documentation):

   ParseResult ok = doc.Parse(json);
   if (!ok) // ...

fix Document::Parse(const Ch*) for transcoding

To allow for an UTF16-Document to Parse(UTF8),
the Parse() argument has to be UTF8-compatible.

documenttest.cpp: EXPECT_THROW when checking empty allocator (closes #469)

In the MoveConstructor/MoveAssignment tests, a check for a `NULL` allocator
return has been used to check for the correct moved-from state of a Document.
After the merge of #426, the GetAllocator call fails with an assertion,
if the allocator of a GenericDocument is NULL.

Check for the throw, instead of NULL in the move-related tests.
Tested with GCC 4.9 on Linux with C++11 enabled.

Reported-by: @woldendans

Adding coapp definition

Adding usage comment at the top of the file for knowledge sharing