Added support for zooming using the mouse wheel

Switched to Tomi Aarnio's fork of lightgl.js which supports mouse wheel
events
Removed the warning if a browser other than Chrome is used

OpenJsCad seems to run fine on the latest Firefox
Fix error in firefox: 'extensionForCurrentObject is not a function'
Additional boilerplate added to DXF export.  Now, exported DXF files
include HEADER, TABLES and BLOCKS sections as well as entities.  In
addition, a default layer and line style are defined.

This improves compatability with some DXF importers, notably Inkscape.
Merge pull request #16 from tonious/gh-pages

DXF export compatibility fix.