Add glfwGetTimerValue and glfwGetTimerFrequency

This adds raw timer access to the public API and builds the
floating-point time functions on top.  It also makes the GLFWuint64 type
public.

docs/input.dox

@@ -554,6 +554,21 @@ glfwSetTime(4.0);
 
 This sets the timer to the specified time, in seconds.
 
+You can also access the raw timer value, in 1 / frequency seconds,
+with @ref glfwGetTimerValue.
+
+@code
+GLFWuint64 value = glfwGetTimerValue();
+@endcode
+
+The frequency of the raw timer varies depending on what time sources are
+available on the machine.  You can query its frequency, in Hz, with @ref
+glfwGetTimerFrequency.
+
+@code
+GLFWuint64 freqency = glfwGetTimerFrequency();
+@endcode
+
 
 @section clipboard Clipboard input and output