readertest: Suppress "dangling-else" warning on GCC 7 and later

GCC 6.x doesn't yet support this warning flag, as reported by
@ragnar-ouchterlony.

Fix warnings on GCC 6 and later (closes #666)

document typo fix and .gitignore update

* document.h
  * suppress -Wterminate on GCC 6.x and later
  * simplify warning handling
* schema.h
  * drop RAPIDJSON_NOEXCEPT from GenericSchemaDocument constructor
    (calls RAPIDJSON_NEW anyway)
  * simplify warning handling
    (avoids RAPIDJSON_POP mismatch on Clang)
* encodingtest.cpp, istreamwrappertest.cpp
  * work around -Wdangling-else
* readertest.cpp
  * suppress -Wdangling-else

Doxyfile.zh-cn is a generated file that we need to ignore.

Fix undefined behaviour

UBSAN gave in Value.IsLosslessFloat:
include/rapidjson/document.h:981:38: runtime error: value 3.40282e+38 is outside the range of representable values of type 'float'

note that std::numeric_limits<uint64_t>::max() and
std::numeric_limits<int64_t>::max() aren't exactly representable in a
double, so we need to be strictly less to be definitely lossless

UBSAN gave during Value.IsLosslessDouble test:
include/rapidjson/document.h:955:42: runtime error: value 1.84467e+19 is outside the range of representable values of type 'unsigned long'

UBSAN gave in Regex.Unicode test:
include/rapidjson/encodings.h:157:28: runtime error: shift exponent 32 is too large for 32-bit type 'int'

UBSAN gave during Reader.ParseNumber_FullPrecisionDouble test:
include/rapidjson/internal/strtod.h:149:11: runtime error: shift exponent 46 is too large for 32-bit type 'int'

UBSAN gave:
runtime error: division by zero

maybe these tests should just be deleted?

UBSAN gave:
runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long'
runtime error: signed integer overflow: -9223372036854775808 - 2 cannot be represented in type 'long'

UBSAN gave for test/unittest/itoatest.cpp:87:
runtime error: signed integer overflow: 4611686018427387904 * 2 cannot be represented in type 'long'

UBSAN on Clang/Linux gave:
runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:43:45: note: nonnull attribute specified here

UBSAN gave issues with the typeless Schema:
runtime error: reference binding to null pointer of type 'rapidjson::GenericSchemaDocument<rapidjson::GenericValue<rapidjson::UTF16<wchar_t>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >, rapidjson::CrtAllocator>'
and
runtime error: member access within null pointer of type 'AllocatorType' (aka 'rapidjson::CrtAllocator')

UBSAN gave "runtime error: index 13 out of bounds for type 'const uint32_t [10]'"

specifically, "expression with side effects has no effect in an unevaluated context"

clang advises: "note: silence by adding parentheses to mark code as explicitly dead"

switch Clang from broken Ubuntu PPA to upstream binary release

This should fix the Travis clang builds, since the upstream LLVM apt
repo is down.

npm docs

Fix cmake build with new gtest reference

Allow options for writing and parsing NaN/Infinity

This adds kWriteNanAndInfFlag to Writer to allow writing of nan,
inf and -inf doubles as "NaN", "Infinity" and "-Infinity",
respectively, and kParseNanAndInfFlag to Reader to allow parsing
of "NaN", "Inf", "Infinity", "-Inf" and "-Infinity". This is part
of issue #36, adding optional support for relaxed JSON syntax.

npm compliant package.json