added performance boost for text rendering for Chrome & Firefox,

while retainining compatibility with Opera & custom text settings.

src/Core.js

@@ -18,6 +18,7 @@ function html2canvas(el, userOptions) {
         renderViewport: false,
         reorderZ: true,
         throttle:true,
+        letterRendering:false,
         renderOrder: "canvas flash html"
     });