Increase maximum density of 2D barcodes to 64 blocks/in. (#78)

This affects the minimum size of 2D barcodes.  Even though you can print
at this density, it may be problematic depending on printer quality or
scanner optics.

glbarcode/Barcode2dBase.cpp

@@ -33,7 +33,7 @@ using namespace glbarcode::Constants;
 namespace
 {
 
-	const double MIN_CELL_SIZE  = ( 0.0625 * PTS_PER_INCH );
+	const double MIN_CELL_SIZE  = ( 1.0/64.0 * PTS_PER_INCH );
 
 }