Fix #1017 allOf keyword fail with Writer handler

Gave up using static binding for null handler, because it cannot be
used with arbitrary handler type. Change `OutputHandler handler_` to
pointer type.

GenericStringRef: move assert out of expression

travis-doxygen.sh: upgrade to Doxygen 1.8.13

 * Upgrade to the latest Doxygen version 1.8.13
 * Drop unused variable DOXYGEN_BIN
 * Reenable --single-branch (travis-ci/travis-ci#5225 is closed)

Travis: Switch to Ubuntu 14.04 (Trusty)

Improve handling of NULL strings

ParseResult: improve bool conversion and add operator!=

Tutorial: fix typos in examples and broken links

 * Safely assert upon passing NULL string without length
   (requires usage of RAPIDJSON_ASSERT within an expression)
 * Allow using a NULL string together with an explicit length 0
   (GenericStringRef, GenericValue::SetString, ...), see #817
 * Add GenericValue::SetString(StringRefType, Allocator&) overload
 * Add tests for the various cases

 * Use safe-bool idiom for boolean conversion to avoid accidental
   misuse of ParseResult values (closes #989)
 * Add operator!= to support more comparison expressions
   (previously silently/erroneously used operator bool)

In the move example, the code uses `contacts` when the diagrams use
`contact` (no 's')

The code in the example:
```    Value contacts(kArrayType);
    // adding elements to contacts array.
    // ...
    o.AddMember("contacts", contacts, d.GetAllocator());  // deep clone contacts (may be with lots of allocations)
    // destruct contacts.
```

Intel C++ Compiler preprocessor guard for #pragma intrinsic - diyfp.h

Fixed #985 : Unittest failed with MinGWx64. And few small improvement…

Improve readme.md

Add alt text for images
Use https whenever possible
Update URLs
Use tools.ietf.org for RFC7159
Correct indent for sublists
Trim trailing whitespaces

Fixed #985 : Unittest failed with MinGWx64. And few small improvement were done while looking for mistakes.

Problem was because of Windows uses backslashes '\', not Unix '/'

Fix patternProperties & additionalProperties lead to ASSERT

Fix #825

Fixup #964 by forwarding copyConstStrings recursively

As reported by @Llerd in #962, the `copyConstStrings` parameter
has not been forwarded recursively to the constructors of
object members and array elements.

Storage class is not first - encodings.h

warning #1879: unimplemented pragma ignored: #pragma intrinsic(_BitScanReverse64)
warning #1879: unimplemented pragma ignored: #pragma intrinsic(_umul128)

If storage class is not specified as first in declaration then Intel C++ Compiler 2017 generates message:
message #82: storage class is not first

GenericValue::CopyFrom: add option to force copying of strings

Copying the result of an in-situ parsing into another value/document
currently requires that the original buffer - still holding the strings
from the parsing, outlives the  destination object as well.

In order to obtain a "full" copy of a GenericValue, this commit adds
an optional flag `copyConstStrings` to `CopyFrom`, which then forces
to take a copy of all embedded strings in the source value.

This solves the problem discussed in #962.

typo

See #930

Add convenience method Key(std::basic_string<Ch> const&) to Writer

Add fallthrough attribute to suppress GCC 7.1 warning

typo