Revert "snprintf.cxx: Assume std::vsnprintf() and std::vswprintf() are always
available." The vswprintf() function is still not usable on Windows.

This reverts commit 0d1a7ff45f5aafaab15cc34ae33cf4edb4ece084.
Fix MinGW compilation issues. Mention MinGW support in README.md.
stringhelper.h: Use constexpr for is_negative().
tracelogger (TraceLogger).h: Move message and logger arguments in ctor.
Use C++11 moves instead of swaps where possible.
hierarchy.cxx: Use emplace() into a map instead of insert().
Use nullptr instead of zero or NULL.
Merge pull request #21 from wilx/master

Merge.