Merged joystick functions into input module.
Merged time functions into input module.
Fixed OS X scroll X-axis inversion.

When natural scrolling is disabled on OS X, the X-axis of the scroll
offsets is inverted compared to the direction on Windows.  The X11
scrolling directions are unspecified and so have been aligned with the
Windows port.  Natural scrolling inverts both axes on both OS X and X11,
so the issue remains when the feature is enabled.

This inverts the provided X-axis scroll offset, making "unnatural"
scroll data align with the Windows and X11 ports and "natual" scroll
data be fully inverted and aligned with its counterpart on X11.

Fixes #239.
Updated comments for DLL macros.
Documentation work.