Removed compiler warning about unused variable 'kMinExp'

Howto installation of json_fwd.hpp (fixes #923)

- Builds and runs test cases from individual sources rather than from amalgamation.
- Checks whether amalgamation would create the same single-header file as checked in.

Also fixed some warnings from GCC.

Fix a typo in README.md

fix sfinae on basic_json UDT constructor

Floating-point formatting

Updated README.md to explain how installation of json_fwd.hpp can be achieved as part of install step.

JSON Merge Patch (RFC 7396)

Avoid compiler errors when performing SFINAE checks on basic_json
and incomplete types.

This is an attempt to fix #360. The algorithm produces
decimal representations which are guaranteed to roundtrip
and in ~99.8% actually produces the shortest possible
representation. So this is a nice compromise between using
a precision of digits10 and max_digits10.

Note 1:

The implementation only works for IEEE single/double precision
numbers. So the old implementation is kept for compatibility
with non-IEEE implementations and 'long double'.

Note 2:

If number_float_t is 'float', not all serialized numbers can
be recovered using strtod (strtof works, though). (There is
exactly one such number and the result is off by 1 ulp.)
This can be avoided by changing the implementation (the fix
is trivial), but then the resulting decimal numbers are not
exactly short.