Adds two missing timing functions from the library

sys_elapsed_milli and sys_get_last_frame_length are both useful functions.
Adds missing function get_fps

So that we can have an FPS counter, of course!
Merge pull request #9 from tivec/add-get-fps

Adds missing function get_fps
Merge pull request #8 from tivec:

- Adds two missing timing functions from the library.