Full UTF-8 support

• Support embedded 0s in strings (since UTF-8 allows them).
  • If you use c-string methods, or std::string::c_str(), you can still have problems.
  • Note that this support has a price: keys are now limited to 1 billion characters (2^30).
• Add feature to allow single-quotes in input JSON (since JavaScript allows them).
• Propagate rarely-used StaticString thru copy-ctor.
• Let JSON_ASSERT throw std::logic_error instead of std::runtime_error.
• Skip Python tests by default, and run C++ unit-tests only for changes.
• Ignore cmake-generated files for in-source builds.
• Add tests. Fix other tests.
• Remove unused code (JSON_VALUE_USE_INTERNAL_MAP).
• Add rejectDupKeys feature.
  • Now part of strictMode.
• Fix minor build issues for VisualStudio.
• Fix compile error for VS2013, plus warnings.
• Add operator[] to Builders.
• In Builders, fix ::validate() (which was always returning true).
• Fix UTF-8 zeroes bugs in previous patch-versions.