After 1.0: Multithreaded saving.

Feels like the OpenXcom on steroids)
Dramatically decreases duration of saving in many situations on multi-core systems.
* Geoscape autosave: I did not feel the difference(
* Battlescape autosave: reduction of the duration of saving by 40-50%.
* Geoscape saving: instant saving.
* Battlescape saving: instant saving.
* For one-core systems, anyways, will be only one thread.

The first time, instant saving is very surprising;)

Disadvantages:
* Added dependence of "boost\thread.hpp".
* Debugging of multithreading applications (sometimes) can be difficult.