Make USE_INT64 compile under Visual Studio 2008

 * Visual Studio or more precisely: MSVC++ 9.0) is not C99 compatible
 * So define the elements from inttypes.h on that compilter
 * Also add wrapper around inttypes.h for other compilers: may be
   needed because of strtoimax function interface
Raise VC warning level to /W4

"Fix" the warnings, mostly rewrites for same effect

 * /W3: C4800: forcing value to bool 'true' or 'false'
   The 1 code line is now a bit more verbose, but no negative effects
 * /W4: C4702: unreachable code
   For MSVC remove a __assume(0) following a failing assertion. This
   may potentially change optimizer results(?)
 * /W4: C4127: conditional expression is constant
   Make some preprocessor definitions more verbose.

For the last point, add the following:
 * PICOJSON_VCWARN_OFF(n) and PICOJSON_VCWARN_BACK to suppress some
   MSVC warnings (only C4127 for now)
 * PICOJSON_PPBLK_BEGIN and PICOJSON_PPBLK_END to make the PP block
   technique easier to read
 * PICOJSON_FOREVER Equivalent to while (1)

And modify PICOJSON_ASSERT to suppress C4127 when evaluating its argument
Take different approach to MSVC warning suppr.

Instead of defining several new macros to suppress C4127 at specific
points, suppress it globally. And suppress the C4702 around
__assume(0) instead of removing that line.
Clarify <inttypes.h> workaround

Keep the workaround-definitions in the global name-space, but make
clear that this happens only on old compilers.
Position __STDC_FORMAT_MACROS correctly

__STDC_FORMAT_MACROS is only used when including <inttypes.h>, not for
<errno.h>.
Use correct C99 value for __STDC_VERSION__
Remove all MSVC workarounds from global namespace

 * Fix how the problem is detected: checking __STDC_VERSION__ always
   fails because a C++ compiler never defines that even if the C99
   library is available. So fall back to detecting specific "broken"
   compilers like MSVC++ <= 9 (compiler version <= 15.00)
 * Prepare all code for addition of more "broken" compilers via extra
   #elif statements
 * Add a typedef picojson::int64_t (instead of the suggested int_t)
   because the name is more descriptive and people are by now used to
   int<width>_t.
Merge remote-tracking branch 'olafmandel/VC_compile'
ignoring test folder.