* Added testing with std=c++17 for latest compilers
Added running latest compilers with -std=c++17 option to CI the test matrix,
Updated cmake configuration to allow passing c++ standard on the command line.
* attempt to fix appveyor break
* added clang6.0, removed c++17 tests for clang 5.0
* commented out tests for clang with c++17 die to issue #695
* Addresed comments
* CI updates:
Travis: use cmake 3.7
Appveyor: Add .appveyor.yml config, test VS2013/VS2015/VS2017 x86/x64 Debug/Release.
* Drop VS2013 support from AppVeyor.
