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.
Merge pull request #95 from shurcooL/master

Added support for Cocoa precise scrolling deltas.