Implement logging framework.

Not hard to use:
```C++
using You::Utils::Log;
Log::debug << L"hello";
Log::debug << f(); // strict
Log::debug << std::bind(&f); // lazy

Log::setActiveLevel(You::Utils::LogSeverity::WARNING);
Log::debug << std::bind(&f); // doesn't get called.
Log::warning << std::bind(&f); // gets called.
```