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...

"make fuzz" creates a simple executable that de-serialises stdin
and re-serialises to stdout.
"make fuzz_testcases" extracts the smaller json test cases into
a testcases directory.

The library can then be fuzzed as follows:
    CC=afl-clang-fast make fuzz
    make fuzz_testcases
    mkdir out
    afl-fuzz -i testcases -o out ./fuzz

- removed m_final member
- parse_internal now takes a reference to a basic_json object to fill
with data
- header is automatically regenerated if re2c file is changed

Addressing:
```
+ make json_unit
g++ -std=c++11  -Wall -Wextra -pedantic -Weffc++ -Wcast-align -Wcast-qual -Wctor-dtor-privacy -Wdisabled-optimization -Wformat=2 -Winit-self -Wmissing-declarations -Wmissing-include-dirs -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wshadow -Wsign-conversion -Wsign-promo -Wstrict-overflow=5 -Wswitch -Wundef -Wno-unused -Wnon-virtual-dtor -Wreorder -Wdeprecated -Wfloat-equal  -I src -I test -Dprivate=public test/unit.cpp  -o json_unit
In file included from test/catch.hpp:65:0,
                 from test/unit.cpp:11:
/usr/include/c++/5.0.0/sstream:300:7: error: 'struct std::__cxx11::basic_stringbuf<_CharT, _Traits, _Alloc>::__xfer_bufptrs' redeclared with different access
       struct __xfer_bufptrs
```