Digraph warning

Update json.hpp

Remove duplicate word

mistake

s/to to/to/

To fix compilation issue for intel OSX compiler

o To prevent the compilation issue on OSX with the intel compiler suite.  The
error was found with icpc version 15.0.3.187 where the "__clang_version__" was
not defined correctly, while "__clang__" was.

The original version can trigger a digraph-related warning in VS2015. Is this namespace construct a typo or something deliberate?

massage -> message

Fix "not constraint" grammar in docs

Add documentation for integration with CMake and hunter

https://github.com/nlohmann/json/pull/663#issuecomment-318931884

REFACTOR: rewrite CMakeLists.txt for better inlcude and reuse

Support moving from rvalues in std::initializer_list

- replaced list of pairs by flat list in next_byte_in_range
- implemented early exit in case of parse errors
- reused memory for object keys
- direct calls to embedded objects/arrays for insertions

The rewrite uses more cmake build-in automatisms and build-in generates
variables to allow better generic reuse.
* cmake  files are installed to
``` <install_prefix>/lib/cmake/nlohmann_json/ ``` for best support on
most systems
* include path is set to ``` include ```  for usage as ``` #include
<nlohmann/json.hpp> ```

enable_testing only if the JSON_BuildTests is ON

You can now pass a boolean "allow_exceptions" to the parse functions. If it is false, no exceptions are thrown in case of a parse error. Instead, parsing is stopped at the first error and a JSON value of type "discarded" (check with is_discarded()) is returned.