Some fixes to allow to compile in GNU/Linux
Merge pull request #8 from Zardoz89/develop

Some fixes to allow to compile in GNU/Linux
Fixed crazy rotations

Now adding a rotational force will not compound. AddRotationForce now
checks to see if the rotation force already exists and returns if it
does.
Fixed SDLSys crashing when releasing certain keys

SDL keycodes go above the 256 ascii key set. This caused a crashed when
trying to set the key code as released.
Hack to slow down SDL timing

SDL timing is based on MS whereas update code happens at sub-MS timing
(physics specifically). By divide it down to microseconds it has the
effect of sub-MS but it really isn't.
Typo and MinGW fixes

Added some CMake checks for MinGW to use SDL over Win32/

Fixed a typo in KeyboardEventSystem.
Fixed SDL::GetDeltaTime()

GetDeltaTime() now properly stores the last time to use when compute the
current delta.