Style links depending on file format

修改外部链接，email链接，pdf链接的样式

/* external links */
a[href^=http://]{
padding-right:20px;
background:url(external.gif)no-repeatcenterright;
}
/* emails */
a[href^=mailto:]{
padding-right:20px;
background:url(email.png)no-repeatcenterright;
}
/* pdfs */
a[href$=.pdf]{
padding-right:20px;
background:url(pdf.png)no-repeatcenterright;