Provided Basic Support for -l N option on windows platform.

I have implemented basic support for **-l N** on windows platform.

Tested with platform==msvc and platform==mingw.

Values returned from OS are filtered with basic filter, to minimize load variation caused by short time between consecutive calls.

I have tested it on 2 core machine with HT and 4 core one, and CPU load in Task Manager matches one provided in -l option.

Any suggestion and improvements are moe than welcome.


It probably should, so I think we can:
1. Multiply current result `* GetProcessorsCount()`
2. Provide two variants of `-l` flag, one for limiting POSIX compatible load, and second for limiting relative load. Maybe `-l` and `-L`?

I implemented patch so if we call ninja -l0.8 on multicore system it will be still usable for web browsing, text editing and other less CPU intensive tasks. POSIX compatible way makes it harder to share scripts between different machines.

@martine What do You think about extra flag?