Fix reload bug

When doing page reloading (Ctrl+R in IE8, for example), since in
main.js#L102 the element value is set and not cleared during the
reloading. So the second time main.js#L100 will evaluate to false, thus
no placeholder is rendered. Instead, the input will have the place
holder value as the actual value. This commit fixed that.
Better compatibility
Merge pull request #50 from ywen/master

Fix the bug where reloading the page will turn the placeholder text into actual value of the filed