add forwarding references to json_ref constructor

fixes #805

Add transparent comparator and perfect forwarding support to find() and count()

Already covered by AppVeyor

Instead implement @gregmarr's PR comments that perfect forwarding should be used. Also cleaned up cpp language standard detection.

Hopefully, #764 fixed this.

Simplified istream handing #367

Copied from solution to https://github.com/nlohmann/json/issues/464

This avoids unnecessary string copies on often used
find().

Error : 'identifier "size_t" is undefined' in linux

Fix Visual Studio 2017 warnings

Fix warning C4706 on Visual Studio 2017 - fixes #784

Set GENERATE_TAGFILE in Doxyfile

Lots of tests have this warning.

Also moved out of for loop, doesn't need to be done every loop.

This causes test-udt to crash due to bad iterator