The resulting install tree, when tests are enabled, looks like this:

```
.
├── cmake
│   ├── nlohmann_jsonConfig.cmake
│   ├── nlohmann_jsonConfigVersion.cmake
│   └── nlohmann_jsonTargets.cmake
├── include
│   └── nlohmann
│       └── json.hpp
└── test
    ├── bin
    │   └── json_unit
    └── data
        ├── json_nlohmann_tests
        │   ├── all_unicode.json
        │   └── bom.json
        ├── json.org
        │   ├── 1.json
        │   ├── ...
        ├── json_roundtrip
        │   ├── roundtrip01.json
        │   ├── roundtrip02.json
        │   └── ...
        ├── json_tests
        │   ├── fail10.json
        │   └── ...
        └── json_testsuite
            └── sample.json
```

It has the property that you can invoke the test binary from the
root of the install tree and the tests work correctly (you no
longer depend on the test binary being run inside the source
tree).

If tests are disabled, the entire `test/` subtree is omitted.
Notice how that yields exactly what you want for using this
library in other projects.

I do not believe I need to update travis due to this change, as the
evil Makefile continues to do in-tree builds. I expect I'll find
out soon enough.

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.

Implement additional integration options

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.