Added a2_GetTicks() and a2_Sleep() to the public API

Added a2_Sleep(), which attempts to sleep for the specified number
of milliseconds. Win32 and pthreads implementations.

Turned a2_GetTicks() and a2_GetMicros() into normal functions, and
made a2_GetTicks() part of the public API.

Switched to the new timing functions in a2play and the test programs,
increasing the poll timing from 1 Hz to 10 Hz where applicable, and
eliminating the SDL dependency from tests that don't really need SDL.