css banner image, style fix, naked excerpt

Change the banner to css background with linear gradient so it will be
in the same style the rest of the blog is.

Add some style fix to many pages.

The index now will only show naked html from posts.

modified:   _includes/banner.html
modified:   contact.html
modified:   css/blog.css
modified:   index.html
modified:   prospective-student.html

Signed-off-by: Yishen Miao <mys721tx@gmail.com>