Add missing include and CMake option

[Facon]

Hello,

I am considering using this in a professional project. I notice that some small improvements are needed for this nice library.

First, on Linux with gcc, all warnings and pedantic activated. I get the following error about a missing iterator header.

Also, I tried to compile the tests, but the option is not properly linked to the test folders.

With this pull request, both issues should be fixed.