fix(draw): revert to Arial as default font

This partially reverts commit 891cf4ac0cd262295e4bfbf99cf8ac67ece3d851
which set `sans-serif` as the default font.

Setting it back to Arial solves SVG export and rendering issues,
especially on platforms (Windows, Mac) where the default font
metrics differ highly from Arial.

NOTE: The font can still be easily overriden if integrators wish
to do so in order to give their users a more native look and feel.

Closes #819

lib/draw/TextRenderer.js

@@ -11,7 +11,7 @@ var MIN_TEXT_ANNOTATION_HEIGHT = 30;
 export default function TextRenderer(config) {
 
   var defaultStyle = assign({
-    fontFamily: 'sans-serif',
+    fontFamily: 'Arial, sans-serif',
     fontSize: DEFAULT_FONT_SIZE,
     fontWeight: 'normal',
     lineHeight: LINE_HEIGHT_RATIO