Added the download link for Unihan database

It is easier to have a direct download link than the project page.
Fixed issue with codepoints pass U+FFFF

The original hexToUnicode can only convert Unicode code points
U+0000 to U+FFFF. This is due to the short coming of Unicode literal
with a lower case "u" (e.g. "\u1234").

This commit changed the literal to use upper case "U"
(e.g. "\U00012345") to map later code points.