Changed scrollWheel to use precise scrolling deltas when available.

Use precise scrolling deltas only on supported versions of OS X;
hasPreciseScrollingDeltas, scrollingDeltaX, scrollingDeltaY are
available in OS X v10.7 and later.
Replaced a single occurence of a tab character with 4 spaces for
consistency in src/socoa_window.m.
Updated extension headers.
Merge pull request #95 from shurcooL/master

Added support for Cocoa precise scrolling deltas.
Updated extension headers.
Enabled inclusion of stddef.h on all platforms.

The OS X gl.h defines section macros that prevents glext.h from
including stddef.h, leaving it undeclared when it's later used.
Added explanation for #error.
Fixed #if-guard logic and signature for hasPreciseScrollingDeltas.
Merge pull request #102 from shurcooL/patch-1

Fixed OS X precise scrolling delta logic.
Fixed glfwCreateWindow return value semantics.
Added pointer swap macro for callback setters.
Replaced selector test with NSAppKitVersionNumber.
Simplified source file comment headers.
Updated change log.
Fixed window position being incorrect on some WMs.
Added clearing of callbacks on terminate.
Fixed original video mode being overwritten.
Removed cursor centering XFlush hack.
Formatting.
Expanded section on X11 and Mesa packages.
Build documentation improvements.