This introduces a clear separation between test data and test
binaries. Test data is moved into test/data, and the test binaries
move into test/src. A new CMake script specific to building the
tests is introduced in /test to slightly clean up the toplevel
one.

As well as tidying things up, this makes the next step trivial...

There exist lots of json libraries, and project/target names must
be globally unique. If someone integrated with this library in a
particularly stupid way, using a generic name like "json" might
cause a problem.

I'm not sure that using a variable for target names really helps
with clarity. Unlike paths, target names aren't really something
you change. In a sense, targets are themselves a sort of variable,
so having a variable to name a variable seems just a bit gnarly.

Generate the config files of the package in CMAKE_CURRENT_BINARY_DIR
instead of CMAKE_BINARY_DIR because the later will refer to the main
project's binary directory if the library is included in the project
using the add_subdirectory command.

- define the project's version in the cmake listfile
- create a config and config version file for the cmake package
- install the json.hpp file and the package's targets, config and
config version files

Define the library as an interface imported library so other targets
can use the library as a dependency and use the interface properties
of the library.

- additionally, switch off optimization flags to maybe allow build to
complete on AppVeyor