Update social icons on homepage and WOTD page

Updated Google+ and Facebook icons with the official ones (https://developers.google.com/+/branding-guidelines and https://www.facebookbrand.com/).
Uploaded an improved icon for Blogger.
Removed border-radius and border for these icons, to preserve more the original shape.
On WOTD page, created a set of flat icons for email and RSS, in order to look similar to Facebook's icon style.